What size UTH do you have? A" X B" You may need to get a bigger one but never fear the other one can be used under the cool side to help maintain an ambient air temp around 78 to 80 degrees. You just need to add an inexpensive on off type thermostat to control the second mat at a lower temp than the larger mat!
What are the ambient air temps?
I know some of the UTH mats are designed to never go over a certain temperature. Is yours one of these?
To get the glass on the bottom of the tank up to around 92 degrees you will need to get the mat to around 95 degees or so as the glass dissipates the heat into the room the tank is in pretty fast.
If you haven't yet, you could look into covering the top of the tank in Aluminum foil. Cut out a small section to allow fresh air to enter or poke a dozen or so 1/8" holes in the aluminum foil on the cool side.