I found all the gems and got 3 heads to turn and then the fourth head. I painted in one eye, clicked A to insert a gem, got the message that it "clicked into place" but the gem doesn't show up and it's not in my inventory and the head doesn't turn. So no there are no more gems, and I'm screwed.
So I quit, reload. The puzzle has reset. I go and get all the gems again. Start with that head, the exact same thing happens, it says it clicked into place but it doesn't go in, and I can't get the head to turn.