Nassarius Snail acting funny

[NewReefMom]

So I have 2 nassarius snails in my tank and both were eating fine. Like normal, they would dig themselves in the sand then come up as soon as I fed. One still does that but the other hasn't been coming out. I sifted the sand a bit to see if he was dead but he is still alive. I put some food right next to him and he ate but then went straight back into the sand.
Is this normal behavior? Or could he be sick?

 

[jeffchapok]

They are mostly nocturnal. I seldom see any of mine.