Try the misting advice PHEve gave. If that does not work, try placing it in a humidity chamber. A large plastic container with small breathing holes. Put in a warm damp cloth or paper towels and something for it to rub against such as a piece of cork bark. Close the lid and let it stay in there for ~1 hour. Do not place it in the sun. You could place it on top of the terrarium to keep the temps warmer promoting a sauna effect. This should definitely aid the molting process.