IF you're sure he has adequate warmth, & you're sure his rodents aren't too big (try downsizing first & make sure he has adequate time to digest) then maybe his digestion just isn't as good as it should or could be? I've had one snake in the past that had very poor digestion- for whatever reason, hey, it can happen. All snakes aren't perfect any more than all humans are- many or all of us have some weak areas, so if this snake has digestive trouble with thick furred rodents, he might do better being fed mice (not rats), or something else (hairless, maybe) ?