To be clear. Is it 90F on the heat mat, or is the thermostat set to 90F?
What size tank? What's humidity, etc?
We need more info, but I would probably move the heat lamp closer to the middle to warm the air, or get a second one. I also recommend using ceramic heat emitters of lamps/bulbs as BP's like dark at night and do not need a temp drop. Also, is the lamp controlled by a thermostat? Unless it's a tiny bulb and not capable of producing heat, you should have all heating elements controlled by thermostat, not just the UTH. However, very good the UTH is controlled by one.
Hot side: 90F
Ambient/middle temps: 80-84F
Cool side: 78-80F
74F anywhere is cold.
Please give us more information so we can better help you.