How hot is your hot spot?
Just because a thermostat is set to 90, doesn't mean that's the temperature inside the tub.
If it's low, you might have to set the T-stat to a higher temp to achieve the correct one inside the tub. For example, I set my Herpstat to 94 degrees to achieve a 90 degree hotspot inside the tub.
And also, the heat pad will not affect ambient heat of the cool end.
It might influence a little, but not significantly enough. Especially if your tub is a stand alone tub and not insulated in a rack or something. Your cold side ambient will be whatever your room temperature is. So you can either try to insulate your tub or bump up the temperature in your room.