I had the same problem with my Metamorpho. What you are dealing with is a stiff joint.
I was able to fix it by taking a screw bit extender for a drill and inserting the hex piece into the hole in his torso that corresponded to that leg. I gently rotated it until it turned freely and then continued to turn it until it was loosened to the point I wanted it at; where the leg would move but was still far from floppy. I had to do the same to his left arm socket.
If the leg moves but feels like it turns in increments the joint still isn't turning, but the hex peg is slipping in the socket. That's no good as you will weaken the figure. Insert the tool and repeat.
Anyhoo, good luck, that bad boy is one awesome fig!
PS: I also had to boil the clear leg to straighten it out. Yeesh!
I had to do the boiling too and I will have to do the same thing to his legs for mine are doing the same damn thing