My 276 pickup was slipping and finally "stopped" locked up. After taking of the pickup guards we found that the cam bearings were wornout, some had no roller, others locked up. They are hard to get to for an old guy. I just put 6 new rollers into the supersweep and it seems to be freed up and working OK.
You might first see if the pickup moves freely or locks up before adjusting the slip clutch.