Most are from not having springs and stuff changed .
The recent one is the indexer not returning all the way, short 1/4" before pawl flicks back to final place, so will not index for next load.
Checked for burrs, changed return spring. On the pawl spring, when pushing the pawl in and out it is not a smooth action but catchy.
RE: Indexer return
Remove the bolt that attaches the index lever to the frame, clean it and the index lever up, regrease the parts and reassemble. Be sure the leg of the index return spring is resting on the screw, or on top of the primer rocker arm on late machines.