i think the pickup slip clutch is the likely cause of this issue. it's designed to slip when there's an overload to prevent damage, but if it's too loose, it will stop spinning even under normal load. it's usually a matter of tightening the nuts on the clutch springs a turn or two at a time until it holds the load so i suggest you check your owner's manual for the correct procedure on how to do it that. just make sure you don't over-tighten it, or it won't slip when it needs to!! a less common but possible cause could be a broken shear bolt, though that usually results in a more sudden and complete stop of the pickup.