RicOlie_2 said:

In ROTJ though, he wasn't bright enough to realize that he it was Luke who levitated him...I don't know, sometimes he's smart, but other times he just seems a bit slow.

It was probably lazy writing, but to be fair to C-3PO he wasn't around Luke when he was levitating rocks and things with Yoda. So even if R2 had told him about Luke's full abilities, he probably wouldn't have believed it.

This brings up another issue with Luke's powers. Han by his own admission doesn't believe in the Force, so when Luke levitates C-3PO it seems strange that we get no reaction from Han. He actually looks quite happy that Luke's sorcery is working, and then immediately makes out with Leia. Exactly how much power does he think Luke has, anyway?