He just told me he used an auto rigging tool, so I'm guessing it's something to do with the rig. It's possible it isn't completely set up when we start but I dunno. I'm still an amateur in animating :/
It happened on my sword animations and he fixed them so the arms moved. I have a feeling there is an extra step that 'we' aren't doing to get the arms moving. Whether they aren't rigged properly, or we didn't set it up right.