1. Is your UTH on the OUTSIDE of the cage? The way you worded it makes it sound like you have it on the inside. It should be on the outside of the cage.
2. Do you have a thermostat hooked up to your UTH? If you don't, you NEED one. That will keep your glass from getting too hot and burning your snake. As it is right now, if your snake were to sit on the glass, it would have a burned belly without question.
3. How thick is your layer of substrate? It shouldn't be more than 1/2 inch thick or else the heat from the UTH will never reach the snake.
Once you have a regulated (with a thermostat) UTH and only a small amount of substrate on top of the glass, your temps should be where you want them. no more than 92* on the top of the glass. That will probably put your temps around 88-90* on top of the substrate which is perfect hot spot.