advice if buying glass in Murano/Venice

It is possible that the glasses you bought were actually made in Murano and the others were imports from China. There is a lot of glass that is imported and a much cheaper price is one of the distinguishing characteristics.

L'Isola ( by Carlo Moretti) sells beautiful handmade blown glasses pieces in Venice.

It's possible but we looked carefully and could see no discernible differences.

Did both the glasses you bought and the imitations have the Murano trademark on them? The trademark which guarantees genuine Murano looks like on this link:- http://www.muranoglass.com/en/

I just checked and the ones we bought and they do not have that Murano trademark. Very disappointing
They told us they were made on Murano.

Murano is slowly dying, put out of business by Chinese imports. The honest stores will mark the Chinese products with an Ex sign meaning that they are Ex Urbe, or out of town. You cannot tell the difference except for the price. Best to ask a local such as your hotel who sells real Murano glass. Apprentice pieces, all excellent, are sold on the Venice to Croatia catamaran, Venezialines. They must also be sold in Venice somewhere too. A real apprenticeship lasts years. The exquisite top end chandeliers etc tend to be real, and priced accordingly. The more you pay, the more likely it is Murano. The big stores will ship big pieces for you.

Wow 55 euros each they must be very nice glasses

To add insult to injury, they told us we would get back the value-added tax and gave us an orange form that says
TAX FREE (there is no other name of a company), which customs stamped at the airport and we mailed as told, but
we have never gotten the VAT back. I photographed the whole form before mailing but do not see any way to contact
the tax-back company.

Items made in China usually have "PRC" stamped on them.