Flexwatt on AP cage not hot enough

Alright, so, I finally got my new AP T3 all set up. The flexwatt is taped to the bottom with aluminum foil tape and the Herpstat probe is in the spot that's carved out for the probe, between the flexwatt and the bottom of the cage. I initially set the Herpstat to about 98 degrees F, since that seemed to be the temperature that worked well for the UTH on my glass tank. That was last night. This morning, I checked the temperature (I have an Acu-rite thermometer and the probe is drilled into place right in the middle of the flexwatt hotspot) and it's 82 degrees F. Ok, so I turn the Herpstat up to 104 degrees F. When I checked on it again tonight, the Acu-rite read 84 degree F. What the heck. So I check the Herpstat probe, it seems to be in the right spot. I touched the flexwatt and it's definitely hot.

Are the PVC cages really this resilient to flexwatt heating? The only thing I could think of that I'm doing wrong is that I have the cage propped up off of the surface it's on by about 1.5 inches, because I wasn't sure it would be a good idea to have the flexwatt directly on a tablecloth (I have a tablecloth on this table in order to hide all of the reptile supplies under it. I have a small apartment, I have to be creative sometimes). So...any ideas on this one?

What is the temperature of the room that cage is in?

My T8's have their thermostats set between 98-105 degrees to achieve a 90-92 degree hot spot. Keep in mind that it takes awhile for heat to penetrate the 1/2" thick PVC (which takes a lot more to heat through then 1/8" thick glass does)

The bottom cage in each stack that I have has 8 inches of air space under them, but the rest don't have any air space. The cages with air under them have to be set a few degrees warmer than the cages that don't.

Is the flexwatt taped tightly to the bottom of the cage? Any sag in it will make it heat less efficiently.

Your going to want to check the surface temps with an infrared thermometer.

The hotspot finally reached the right temp! Apparently the trick was to place the cage flat on the table...and close the cage doors. I had them open at the time because the humidity was ridiculous with new, wet coconut coir put into it. My boyfriend was the one smart enough to think about shutting the doors. Well, I feel dumb. Anyway, now I can't wait to move Onyx into her new cage tonight :)

Yeah, if the cage doors are open the temps drop pretty quick. They are amazingly insulated when closed though.