I read somewhere last week that snails may take a few days to acclimate in the tank before they get their groove on.
For example 10 days ago, I put 2 certh and 1 bumble bee nass in on the same day. Once certh got is groove going after 2 days, the other one took 4 days, and the nass is still hanging on the same piece of rock not really moving after 10 days.
The other think is snails I believe are nocturnal, so they may not move much during your waking hours.