When you use the heat gun, hold the trigger down and move the gun throughout the heated area. You might just be measuring in a cold spot. Also, whatever substrate you're using, (newspaper for me) if the paper is lifted off of the tub just slightly, then the temp measured isn't accurate. Measure the tub itself and if it's in the temp range you're looking for, then you'll be fine.

Where is you're accurite probe measuring from?