Some snakes can be pretty clueless about drinking from tall water "bowls"...maybe that's part of the problem. Most milk snakes aren't big enough to eat adult
mice...now I'm not there seeing him in person, so I could be wrong, but they have a small mouth. I can't remember ever feeding mine adult mice...just hoppers.
But what I do with specific snakes that have a narrow gape and a long body (ie. plenty of space lengthwise to fill) is give several small mice of appropriate size
as one meal. No kidding, I have a 32" Texas longnose snake (tri-color, very pretty like a milk snake) and his mouth is so narrow, the most he can manage are
fuzzies or small hoppers. I just fed him today, he had 2 pinks & 3 fuzzies. Now he's happy...and btw, he's over 16 years old. They normally eat lizards in the
wild, & I've had w/c rescues in the past, but he's c/b and I've had him since the pet store he was in (as a yearling) couldn't get him to eat & couldn't sell him.