Has anything in the husbandry changed recently? Or the surrounding area? Maybe he is under the weather and needs a check up at the Vet. (I'd recommend)

You say its spot on, just to make sure, give us some numbers. (you knew we'd ask)
Secondly, how long does after he's swallowed do you offer another mouse?
How long has this been slowly in the decline? How big is he/ how old?

Maybe he is going off feed.

I'm just throwing ideas around here, but if it's enough to bother you, take him to the Vet to be sure.