>>8054406>>8054438>>8054464It's just a problem of the connection, check my pic and I'll explain. If the red rod isn't fully inserted in that black piece of the shoulder it doesn't rotate freely and that's what breaks it. On my copy the hole in that shoulder is way thighter than the other for some reason, so I removed the upper arm piece so I could check if I was inserting it fully and I just attached and dettached the thing several times being extra careful not to twist it. Now it works like a charm, I can remove the arm really easily, it attaches securely nonetheless and rotates freely.
So yeah, the way to fix this is just removing and reattaching the arms several times witout twisting them. I hope this helps.
