Where is your thermostat probe now? Keep in mind that if you have the probe on the heat mat, it will only allow the heat mat to reach the set temp...not the cage! This means that it is going to take a while for the temps in the cage to come up. Think of it this way... if you set the thermostat to 92 degrees, the heat mat heats to 92 degrees and shuts off. The top of the substrate in the cage is only beginning to warm up, so it may still be at 70ish. The heat mat comes back on and heats to 92, then shuts off. The glass is starting to warm up, and the substrate next to the glass. Gonna take a while to heat up the substrate! You may want to set your thermometer probe next to the thermostat probe for a while just to monitor what the heat mat is actually putting out. This is one advantage of having a digital thermostat (helix or herpstat). It will show you the temp at the probe.

Steve