looks like spixi snails. Either they are so young and you got great picture skills. I got eggs from a friend and after hatching there is a period where they look like slugs/planeria. All of a sudden you will see shell development. Unless you have deformed ones and/or somthing is blocking their shell growth. Those are great snails to remove Hydra! Proven!
if all the shell-less snails are about the same size, i would say it is a deformity that happened to all the snails in a certain clutch, and for some reason, these few survived. i can see snail entrails, eeewww!