Starter motor not turning engine

[Bob Culver]

An accumulation of dirt can gum up the engagement. For this reason many books recommend no lubricant but as above I favour thin oil cleaned as necessary. 
Not likely to change in place but the grip of the retainer clip is quite critical. Easily bumped. Too tight and the pinion is reluctant to free and too loose it can wander into mesh underway with curious musical ting ting sounds until it resettles.