Does the door move right away when you press the button, or is there a delay?
We had a problem last year where the garage door opener would run, but the door
would only start moving after a few seconds. Eventually it stopped moving completely
even though the motor would still run.
It turned out that the gears inside the opener (plastic) were worn out and we had
to get a new one. Fortunately, our home warranty covered it, so it wasn't that expensive.