Hi!
Did you already try to open and clean out the entire weighing mechanism? Hardened grease seems to be a common fault and you can check the entire list of possible problems in the weighing mechanism, I can think of:
-hardened grease (easy to solve, very likely)
-broken or worn out component (not likely, not much strain on these things I'd say)
-bad setting of a control (but, is this controlled by a setting, afaik this is purely mechanical)
It's a lot of work, you need to memorize/write down how to put everything back together as you take it apart, but when finished and if the machine works again, the satisfaction will be immense :)
I can't help you any further, as my BG3000 went in the closet because I exchanged it for a BG2000 witch also works with the 3000 system, since the radial turntable gives me more of a 'record playing-feeling' than the beautiful and posh, yet a little sterile and too perfect for my taste BG3000 tangential. It hardly shows the spinning record, one of the great charms of playing LP's!
Again, good luck!