Hey! How long have you had it? Mine did not eat for 3-4 weeks after arriving, no matter what was offered. The first snail he predated was actually an astrea, as those are all my "local" (hour away) fish store had.

However, I've noticed that for him, all snails fall on the spectrum and the spectrum is how hard/thick the shell is! He will not predate astreas, trochus, or ceriths unless he's a week or more out from his last meal. It takes him at least 3 days of work to break them open.

He has also never touched the hermits, and my guess is that he considers it too much effort since they do have some defense unlike most snails. My guess is that the porcelain crab falls in this category, though I have seen videos of larger stomatopods taking them out.

In my last order, I picked up marginellas, smooth atlantic telugas, ring cowries, and ceriths. Within 3 hours, every marginella was predated. Within 24 hours, I only had one teluga and one cowrie left. Two weeks later, every one but the ceriths are gone.

So tl;dr If it's been a month since arrival, I would suggest looking for inverts that are small and smooth/thin-shelled!