Sounds like your UTH is too big for your tank. Typically they're only supposed to cover at most 1/3 of the tank area, on one end. This provides a temperature gradient, with a warm end and a cool end, allowing the gecko to regulate its temperature by moving to the area it's most comfortable in. I'm sorry to tell you that you need to spend more money, but if the UTH is covering the whole bottom of the tank, you need to pull it off and get a smaller one.
BTW, same deal with lights - if you use a light or a ceramic heat element for heating, it needs to be at one end of the tank, for the same reason the UTH does. I have both, but I don't have them on simultaneously. As I said before, in my climate, I usually have the UTH off during the summer, but in winter, I have them on a timer so that when it cycles to night and the light shuts off, the UTH comes on.
Here's hoping this is helpful, and not too expensive for you to handle.
Good luck with your gecko! 