Most likely the Acme nut is stripped in the arm. A common problem. But, unfortunately Von Weiss won't sell you just the nut. Through alot of hassle I had some made. Changing it out isn't difficult. If you want to buy one from me let me know.
The big problem with these units is the Acme Nut stripping out. VonWeise won't sell just the Nut. So, I had some made and want to try to sell them somehow. I'm not sure how to go about it. If you need one, contact me.