For one reason or another, your turtle may not like the current arrangement. He may also bask when you are not there... You cut put a bit of dirt/sand on the basking spot; then you can see whether he ever uses it without you noticing. If you've only had the animal for a short time, he may still be settling in. I would first do the "dirt trap". If he really doesn't use the spot, I'd try a different arragement; maybe add a stepping stone to the ramp to make it easier for him to get out. Also, check the temperature. It should be between 85-90F at the warmest spot. The basking spot should be big enough to provide a gradient.