same thing happened to me. try putting some cork board on the sides and back to add some insulation. also is the top completly a screen? you will want to cover up 3/4 of the top screen with some aluminum foil and tape to keep in the heat and humidity.

even with that mine was still too low. ended up having to switch things up a bit from the standard set up. i put a ceramic heat emmiter over the cool side to bump it up the last degrees into the 80's