This problem also happens when you undo a movement, so the box is hopping to the last position and again the arm isn't following.
I set the arm up exactly the same way as the right side which doesn't have this problem. I've gone through the hypergraph and all seems well, i deleted the parent constraint node and reapplied but no luck. The arm is setup with an IK/FK switch using 3 lots of bones, but i've checked over that and that doesn't seem to be the problem either.
Does anyone have a solution? I'll probably just redo the left side again, maybe i missed a step in the process but i'm pretty sure its done right.
Hope someone has the answer