Heating Issue!

I cant get my temps up High enough in my 3 tubs. I have the T-Stat set at
100F and Im only getting the hot side up to around 86f Max

Suggesations?

Do you have to much space between the heat pad and the tub and maybe need to get the tub closer?

No I dont think so, the tubs fit in the shelf tight, and the tubs are far enough over them, Im using 4" 8watt flexwatt. I did use some alumminum tape over the edges of the flexwatt to keep in place, Also using a Pro Exotics PE-2 Infrared Temp Gun to take the heat readings.

ANy other suggestion or ideas