Are you sanding the edges first? Sometimes the piece is just tooooo slick for the glue to stick. I have a power drill with a grinding bit I run along where ever I plan to glue. Or, just take some rough sandpaper & do it by hand.

Fingers crossed I haven't had that problem yet. I'd be frustrated too.

I always wash, rinse, and dry all the glass before I use it but that's all I do. I know some people rinse with vinegar and/or sand but I don't do that.

I know I've bot some pieces of "colored" glass that turned out to have a colored plastic film coating - there's plain glass underneath. I found out by putting them in the DW and it started to come off. It was a real mess and took a lot of scraping. DH used paint remover on the last one - I was ready to throw it away. I was very disappointed as I thot I had scored some very nice colored glass. I wonder if this is the reason for not gluing well. I know that red glass is more expensive to make so I think that's why they've used the plastic film to make it red or purple.

I used to not sand them, but I'm starting to do that now if it's very slick. If the edges are a little rounded my husband has a grinder that he uses on it sometimes if one doesn't stick. I think I'm going to have to make that a routine practice. I always clean them with alcohol before gluing.