40% isn't horrible -- higher would be preferred, especially when the snake is in shed.

What kind of substrate are you using? From the little experience I have; I have learned that from best to worst for humidity retention is cypress, aspen, and newspaper. I was having trouble with humidity as well. I switched from newspaper to cypress, and that has helped.

You can also put some damp moss (available at most pet stores that have reptile supplies) in the hides when the snake is in shed to help the shedding process.