My garage door opener is not working. This has happened before. The motor runs, but the gear does not move the chain to open the garage door. The first time this happened, I just gave the chain a little push and then it was fine. Now it is doing it again. Only this time I have to give it a big shove, and then it may only open and close one or two times before it stops again. I have oiled the gear and chain and I have tried loosening and tightening the chain with no luck. Any ideas??