Easy fix . The pin in the tube of the hand uses 2 stick like pins. If one of the pins are missing you need to drill a hole and pin back the original pin or make it more stronger. you can put a paper clip at a angle drilled and that will hold you hand.
Sorry for the bad description. The hands are of course connected via ball joints to the corresponding arms which are again connected to the body. The ball joint itself looks like this:
---O---
Here, the --- symbolize the pins of the ball joint and the O the ball joint itself. If you assemble the hole thing you get the following:
Hand---O---Arm
The pins of the ball joints are connected to the arm by sliding the pin into a plastic tube in the arm. On the left side I can see the end of the tube which is covered by the outer rubber body. On the ride side, my arm seems to miss a part of the tube where the pin should be inserted resulting in a very loose connection.