skyjack 3219 will not drive when fully extended

Have the skyjack 3219 that was working fully elevated, I brought it down to move to another location a few feet away, fully elevated it again to move and it wouldn't move. I thought it was from a low battery. I plugged it in overnight to get a full charge, and tested it in the morning. I am able to drive it at ground level and extend it, but it still will not move when fully elevated. What is the problem? I also checked to make sure the pothole protectors were both down and they were, that is as far as I got with troubleshooting.