[Kurt in NJ]

A weak battery can also be the cause---the gear is on a screw thread attached to the shaft with a cushion spring, that spring is attached with bolts with special locking washers

if the battery is weak it won't spin the starter to speed quick enough to screw the gear into mesh with the fly wheel ---ih there is congealed grease on the threads the gear will stick

as has been mentioned, the bolts can break or come out

another thing that can happen with wear the gear can hit the ring gear just wrong and jam, then you put car in 3rd gear and rock to jar it loose