A reason this might be happening is if you don't have a good humidity box, you don't need anything special, just a plastic small container with a hole in the lid or side, inside put wet moss, and place the box on the warm side of the thing you keep your snake in (or the cold side I don't remember!) if you would like to, spray water (not to much) into the humidity box. you can help your snake shed, but only if it has already started! if you are still having problems, you can soak your snake in a container with a bit of water, if it isn't a snake that is meant for swimming make sure you hold it has something to feel secure, make sure the head of your snake isn't being held underwater because it could drown!

I hope you fix your snake shed problem!