I recently bought a 3 ball rolling thunder and have a problem with the bag when in its upright position. The ball on top seems to shift forward when I stand the bag upright. I've contacted Storm's technical support to make sure I have the ball cups in correctly (all of them are straight in a line) and they confirmed that is the correct way. Has anyone else had this issue, and if so, did you solve it? I'd be much happier knowing the balls fit snugly, instead of rolling around. Thanks.