Just added snails; a few still won't come out

my snails seem to trap and realease air to float and move more quickly through my tank. is this odd behavior? they blow out bubbles when they want to fall on their target, how do they get a fatal air problem, if they can ime control bubble flow
 
update: I went back to my "apartment" and checked up on my tank. Of the 6 or 7 trapdoors, only 2 made it :(. I saw my ghost shrimp picking at the opening of one of the shells and I knew it was a goner. Checked a few more and started getting kinda sad. All MTS, except 1, were alive and well. The big ramshorn (not the apple snail kind) was acting kinda strange, lying on it's side, holding onto gravel, but the rest were happily crawling along.

But the real surprise was that I found 3 or 4 baby trapdoors! Still translucent, maybe 2 or 3mm big. I sorta noticed them when I was looking at my ghost shrimp. At first, I thought they were baby pond snails that had sneaked in somehow but I looked closer and saw the shape of their shells. I also found clusters of what I believe were ramshorn eggs on the piece of driftwood. So I'm a bit sad because I lost a good number of my trapdoors but I'm happy that there's a few young ones. I guess the trapdoors did get air trapped in their shells while they were shipping and died.
 
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