I have had similar problems, where for a few MONTHS, it seemed, my turts didn't bask. Now, my turts will bask when no one is around, but when I walk in the room, they're off like two shots! So here's my two cents.
Two things I can think of:
1. It sounds like you have a waterfall type filter. I do too. I have a basking area on that side of the tank, and another on the other. They NEVER bask near the filter, always on the other side. They even stack, rather than bask there. So maybe the water drops do bother them? Try moving it to the other side of the tank? Also, with the waterfalls, water evaporates quickly. One day, they have no problem getting on the dock (on the other side, that is). The next day, I can see it's more of a struggle. So those days where he only comes half out... could the water be lower and making it more difficult? Just something to check. As everyone else said, could just be shyness.
2. I got a wire cutter and cut a circle in the top of my screen. Then I placed the heat lamp right above the circle. This definitely brought my temperature enough. You wouldn't think so, since it was just a screen but... definitely increased basking.
3. Make that three things.

I tried using cardboard to tape around the basking area. Nuthin'. Finally I brought some fake "hanging vine" plants. I suction cupped them to teh wall above water level, blocking the dock. This seems to make them happy: they can "hide" but can still see "danger" (sadly, that's me!) coming into the room. Withut being able to see, the noise may be too freaky for them.
Again, these are just my thoughts. I could be wrong about all of them!