If the sheep came from high ground and are now encountering, say, soft clay, for the first time then shelly hoof is quite likely. We're on heavy clay and see it occasionally. f it causes the sheep to limp I generally clean out the plug of mud, spray in some violet, and leave it alone. It will grow out over the Summer. The hoof wall is much like a thick human fingernail, which will soften if soaked. If you're worried put the worst ones in a shed for a few days for the hoof wall to harden. It'll soften as soon as they're outside again, though.