New Astraea Snails Not Moving...

[Key Equine]

Hello all,
I have a question about a couple of Astraea Snails I picked up the other day. They are my first snails in a new tank, along with a Nassarius snail and three hermit crabs. The other ones are all doing very well, but the two Astraea snails haven't moved since I got them two days ago. I know they are alive because they move a tiny bit in one place and I can see their foot down, but they haven't moved from where I set them down. Any ideas why this would be? I have checked the ammonia/nitrite/nitrate and all are zero.
Also, I acclimated them over about 1.5 hours, bag hanging inside the tank and adding small amounts of water every ten minutes.
Thanks!

Claire