It could be the basking area or the temp of the water.

Make sure the basking area is easy to get on and affixed
for ease. One of my adult males is 2 1/2-3 lbs and I really
had to get creative as he sank everything I used.

The other thing mentioned was the water temp. If the water
temp is too warm there is no need to bask. Water temps
for adults should be around 72-74 and the basking area 85-90.
Try this and see if it helps.

Lastly, he could be very shy and if in an area where there
is alot of movement is afraid to.

Hope this helps
Linda