They hafta be just right for them to switch into a new shell. Like not too big, not too small but just right lol. Mine are picky about theirs but I try to guesstimate how big of a shell they have and will want to upgrade to and try to scavenge when I can at my LFS. Sadly those snails of urs can not flip back up if they fall on their side so they are easy prey for shrimp and crabs =(. But sounds like u might need to feed them a bit more. They sell special pellets just for hermits that I like to treat them to once in a while called Crab Cuisine. Nassarius will have a much better shot bc they can right themselves and they burrow under sandbed for most of the day. Good luck!