If the shaft looks smooth, but there is still some hang up, then the playfield bushing is to blame.
Try reaming out the bushing with a small bottle brush. If the assembly still does not turn smoothly, replace the bushing.
The bushing is a standard flipper bushing with the bottom nub cut off.
1. Drill out the three rivets from the back side of the old bushing with a 1/8" drill bit.
2. Sand, cut or grind off the bottom nub from a flipper bushing, using the damaged part as a guide.
3. Pop rivet the new bushing onto the Airlock bracket.
4. Install in reverse order, and tighten cam so when the power is off the Airlock diverts the ball from the outlane. The set screws should fit into the grove cut into the bottom of the shaft.
5. Use BLUE Loctite on the set screws.
6. Do not oil any part of this assembly.