I have a bit of cuttlebone under the substrate for my snails. Same principle as the vacation blocks. Needed if your waters soft. It helps their shells. I have noticed feeding mine more often gets them to move a lot more too.
When buying them try and get smaller, younger snails. They will cope better with the change of water conditions. I also read that if you make sure you get one that is filling or almost over flowing out of it's shell when closed that it's healthier than one that is sunken in its shell.
I'm pretty sure I've accidentally thrown out live snails too.
it happens to a lot of us. I had one floating, open and not moving for over a week. Finally decided it was dead while doing a PWC
, threw it in the bucket and dumped all the water (and snail) into the laundry tub. Went to wash the sink out with hot water and it moved!! I felt so bad, thought I'd cooked it to death. I ran and dropped it into one of my other tanks and it started crawling away. Didn't act right for a week and I seriously considered putting it to sleep. It recovered though and is still happily living 2 months later.