The humidity is prolly drying up fast because of the heat lamp.
Does the snake use the humid hide during the shed process? When you say tupperware with sphagnum moss, what do you mean exactly?
Humid hides, one both in the cool and one in the warm side during shed should help. A humid hide can be setup as such:

Tupperware bowl large enough for your snake to fit tight along with damp sphagnum moss - covered with a hole large enough for your snake to crawl in an out of either in the top or side.

You can also try a damp towel over the mesh top.