When I first got mine I set the temp at 90 ( I have a zoo med UTH) It stayed at about 130 for about 30 mins, it might take some time maybe if you leave it set. You're trying to get the hot spot to 90 but you have it set for 105? Why not lower the temp you have it set to, to about 92-94? I usually have mine set to about 92 and the hot spot sits to around 88-90. What kind of substrate are you using?