I see where the other reply is going and I agree. If the water temp inside is too high, the turtle may be comfortable enough not basking. try lowering the water temp to 75.
Also if your turtle is nervous, too much activity in the room will keep it in the water. Try covering the basking area so when the turtle is basking, it cant see anyone moving in the room.
These are the 2 most common reasons why turtles dont bask. My turtle would only bask when I left the house. Everyday when I came home and opened the door, I heard the splash of my turtle going into the water. I stopped using my water heater, and left the water at room temps. I do have a canopy hood on the tank, and that traps heat inside. The lowest my water ever gets is about 72F.
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