Train miss, is ticket valid for next one

It depends on the type of train and whether you got a discounted fare.

If you bought a SuperEconomy fare on a high speed FrecciaBianca (from Milan to Venice, let's say), then no. Those fares are non changeable and non refundable.

I didn't have that experience (yet) in Italy, but a few times in Germany and the Czech Republic. If you missed a connection as a result of a delay of your previous train, your ticket is valid for the next train, even if it was a discounted one. Go to an information desk, explain the situation, and they will change it for another ticket or make an annotation on your original one.

Which train did you book? There is a direct train from Montreux to Venice at 8:30 daily, no connection needed, also mentioned to you by Sam in a previous thread. It stops in Domodossola from 10:12 to 10:17, but you just stay on the train.

The next (9:10, connecting) departure from Montreux as suggested on the DB schedule site includes connecting to a few regional trains where reservations won't matter, but then the leg from Milan to Venice is still a faster train for which the cheaper tickets are not refundable or exchangeable.

Train delays within Switzerland are unlikely in themselves and it's also highly unlikely that Italians would flex their rules for the delay of a Swiss train. Allowances in case of train delays are more common within one country or between a few countries (not including Italy) that have partnered on their high-speed service guarantees under the banner of "RailTeam."

RailTeam guarantees only apply high speed train to high speed train.