Take a look at this post in our G&L Knowledgebase forum:
Vibrato Tuning Stability - New S-500 Tribute.
If none of these items to check don't fix your problem and you are the original owner and purchased the guitar from a G&L dealer, contact the dealer.
You have a 10 year warranty with your guitar. If the dealer is local and has a tech department, they might be able to find the issue.
If it is not fixable, they would be able to contact G&L and have it sent to the factory for repair under warranty.
Hope this helps. Keep us informed on any progress in getting this resolved.