It is normal for snails, especially Nerite snails, to not move for several days. They can stay put for up to 72 hours after introduction. Assassin snails have a similar behaviour: it is quite normal for them to just sit still (or even bury themselves into the substrate) and wait for "food" to walk buy. Well-fed assassin snails will show less activity.
If they still have not moved after 48 hours
It is always a good idea to keep an eye on them for the first days. If your snails should not have moved during the first 48 hours, please carefully remove the snail from the tank and have a closer look. Dead snails will smell very bad. If the trap door is not firmly closed, give it a little tap, if the snail does not contract its muscle to seal the door, something is wrong.