Paris transportation

It's possible that you might not have used the tickets properly - using them beyond the 90 minute limit, for example.
Or, if using the bus, trying to backtrack, instead of going in the same direction, which isn't allowed.

The print on the back of carnet tickets can be faint and hard to read. I normally just fold the validated ticket in half once, to tell if I've used it already.

So, you might have tried to re-use one that had already been validated.
Or, a ticket might have gotten magnetized by a magnet in a purse closure.
If this happens, you just have to go to the manned booth and request another ticket for free.

A combination Navigo Pass and some carnets would probably be your best bet for transportation. You'll need 1 photo apiece, and can buy the Navigo at a manned booth in Metro stations. Here's further info: http://parisbytrain.com/paris-train-metro-week-pass-navigo-decouverte/

If you're thinking of buying The Paris Visite Pass, it's overpriced for what you get, but might suit your needs.
You can purchase river boat cruises at a discount - which can be used anytime - on the internet: http://www.vedettesdupontneuf.com/tickets/
You purchase the HOHO bus tickets when you board.

If you arrive at CDG it is 50 or 55 by cab to your hotel (depending on side or river); if Orly then 30 or 35. It is 10 Euro on the RER/metro into Paris and 12.50 (and a PITA ) from ORY by public transport.

The absolute best pass for you is the Navigo Decouverte which runs Mon-Sunday and so will be good for your second week only; it will take you to Versailles, to other places in the Ile de France (it is good for all 5 zones) and will take you to CDG if you are flying from there. For the Sat-Sun you can buy a carnet of ordinary tickets and share them for travel in Zone 1 (Paris) If you are 25 or younger you can get weekend day passes for sat and sun called Ticket Jeune; you can get them for about 3.50 for zone 1 (Paris) or more if you need to include the wider area zones 2-5. The ND is about 23 Euro for the week plus 5 Euro for the renewable pass; you need a head shot which you can just size and print on any paper on your computer (size 25/30 mm) You can buy the ND at the airport RER station or any metro station. You can buy it when you arrive although it won't be usable until Monday -- but if you are taking the RER into Paris then just buy it when you buy those tickets and you are set for the rest of the time.

If a carnet ticket doesn't work then it might have become demagnetized; the information agents at the metro will replace them if not used. Be sure to throw away the ticket when you have left the metro so they don't get confused with good tickets. And if you have magnetic closures on purses or coats be careful; if you demagnetize your ticket when traveling you could get fined for not having a valid ticket.

The river cruises are not covered on any pass; you just walk up and buy a ticket when you want to go; they are cheap.

Don't buy any 'pass' that pretends to include cruises and hohos as well as museums and public transport -- they are hellaciously overpriced and just consist of separate items you can buy for less when you get there. e.g. they throw an overpriced Paris Visite transport pass and a Museum Pass in an envelope and a hoho ticket etc and call it a 'pass'. The museum pass is probably a good idea; you can buy it at any participating museum so just choose one without lines like the Cluny or get it at a tourist office at the airport.

The HOHO bus is not good transportation although I guess you could use it on the weekend; it is slow and you wait a lot. You can buy a 2 days ticket on the bus; no need to decide or buy before you get there. Foxcity has a 2 hour open top bus tour that might be a nice idea if you just want that kind of overview --

I've never had a brand new unused metro ticket become unusable - and I've used many of them over the years. I still say the carnet is THE easiest form of transportation on the metro. Of course using the carnet does mean you have to understand how they work. We took several Uber rides across Paris (use the Uber App) and it was painless and much less expensive than taxis.

We have had tickets demagnetize many times including once having a validated ticket to exit the RER in an outer zone and not being able to use it because it had demagnetized; luckily I was being met by a French person who was able to discuss it with the clerk once he lent me a ticket to exit and get it replaced for the trip home. I might well have been fined because I couldn't prove the ticket was valid as the information was erased.

The ND is so much more convenient than the fiddly little individual tickets and has the virtue of also being stable -- it doesn't demagnetize. For a week trip like the OPs, it is the only sensible choice, especially if the trip involves trips out of Zone 1 such as Disney, Versailles or back to the airport. The OP's return to the airport and trip to Versailles alone will cost about 17 Euro; the ND is about 23 Euro for the week. The carnet if often a wise choice especially if living centrally and walking most places but for a week long stay, the ND is usually the best choice and it can be for shorter stays that involve lots of travel.

Thank everyone so much for your recommendations. As mentioned above, we have had brand new carnet tickets get demagnetized and been unable to exit. Last time I had to use 3 brand new tickets to get my companion out of the metro so we could meet our plane. Anyway, I appreciate the assistance!

"A combination Navigo Pass and some carnets would probably be your best bet for transportation. You'll need 1 photo apiece, and can buy the Navigo at a manned booth in Metro stations. Here's further info: http://parisbytrain.com/paris-train-metro-week-pass-navigo-decouverte/"

For my last visit I was there from Saturday-Saturday. We took a taxi from CDG to our hotel (RER would also work) and then bought a carnet of ticket for Saturday & Sunday. We used the Navigo pass for the rest of the trip, including our trip to the airport. We originally got our Navigo passes in 2008. They are good for 10 years.

I'll echo the Navigo, which will kick in on Monday and take you to Versailles and the airport as well as around town. Your itinerary makes this a good deal for you. Yes you are stuck with tickets for the first day and a half but a carnet ought to do you.

Don't forget to ride the buses, which are frequent and fun.

Comparing transport options
€19 for 2 day Paris Visite Pass for travel on bus, metro, RER in Zones 1-3
€14.50 for carnet of 10 metro trips
€27.15 (including card issue) for Navigo Découverte one week pass (Mon-Sun) on bus, metro, RER in Zones 1-5
Note: Orly & Versailles are in Zone 4, while CDG & Disneyland are in Zone 5.

Verdict
If you are staying just a few days then make do purchasing carnets of metro tickets.
If you are staying for 5-7 days then a Navigo Découverte is well worth considering.
I find I make 3-4 metro trips on an average day when visiting Paris.
Either way, the Paris Visite Pass does not seem to be the best value for travel.

I experienced some issues with Carnet tickets but I think I concluded the problem was some of the gates weren't working rather than my ticket not working.

On maybe my first exit from a metro, I was diligently looking for a place to put my ticket, to get out, and a teen walked past me and just pushed it open. I hadn't considered that. :-)

Is the trip to Versailles on the Navigo Pass a one-time-deal? Can I travel on the RER C to Versailles more than once using Navigo?

Of course, it is a pass. You can go everywhere in the Ile de France on the ND; you can use it to commute from am outer district. You just hop on any train, metro, RER and off you go. You must validate each trip; there is an obvious device to do this.

Thanks! Thought so, but I wanted to check. BTW, would you happen to know the last train from Versailles back to Paris in the direction of the Eiffel Tower? I, for the life of me, cannot figure out how to read the ratp train schedule. I know, it should be such a simple thing to do.

Last train should be 11:55p