When we first added a Mystery Snail to a tank, they did the exact same thing for a couple of days.
It could be that they are stressed or nervous due to the change in environment, just like any other animal.
Don't worry too much as they can hide out for several days before finally 'coming out of their shell'.
It still would be a good idea to take them out and give 'em a sniff once a day to make sure it hasn't died though. Dead snails can cause quite an ammonia spike. Still, only do this once in a day so that you aren't stressing them even more. It may be tempting to poke at their meaty bits to see if they'll move, but don't do this! Our human-ey oils aren't good for their skin and most of the time touching them just makes them stiffen up even more. Be sure not to get any air in their shell putting them back - It won't kill them, but it's not really comfortable either. It could make them float around too, which I imagine is probably pretty disorienting!
If they're still not coming out in a few days and they aren't dead, try putting some soaked cucumber or an algae wafer near them so that they don't starve themselves.
Best of luck! Mystery Snails are surprisingly interesting little guys! Nice to have in a tank.
