I would take Leah's advice and get a temp gun to accurately measure the temps of your substrate surface. Using your hand is not accurate enough.
I was having the same problem in my tank and it was due to the ambient temps being too low. Here's my setup and how I dealt with it.
I have my thermostat probe between the UTH and the bottom of the tank glass and it's set to 96. I then have a digital thermometer probe run into the tank under the substrate resting on the glass over the UTH and it reads 92 - 93.
When I first setup the tank, my ambient temps were in the mid 60s and the surface of my substrate was in the 70s. I then picked up a ceramic heat emitter and a 8.5" lamp with built in rheostat and used that to bring my ambient temps up to 80. Once I did that my substrate surface temps fell in line.
So, things to consider - ambient temps, thickness of substrate, cover 3/4 of your screen lid to help hold in heat if you are not already doing so.
Good luck, let us know how it goes.
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