If the seatpost is the correct side then i would say the threads in the male and female parts, of that pinch bolt, have bottomed out and that is why it wont turn anymore.
Either the male has run into the back of the female, or the Female has run out of thread on the male. To test you could take the two parts out, measure the thread lengths of both, and compare that to the width across the clamp. Quick and dirty would be to stick a washer behind one of them, but unless the washer is hard plastic you run the risk of damaging the paint.