Some of this info couldn't fit onto my headstock repair DVD and I feel it's important for the home user to know.

The most common problem is the quill return spring is no longer working. It can be a break or tear at the cutout on the spring itself, or a reverse bend of the spring from being turned or forced the wrong direction. In either of these cases you will need to replace the spring. It is easiest to replace the spring and housing together, although more expensive. A used unit may be cheaper to acquire.

If the problem is a broken retaining rivet, it may be as simple as replacing that piece, or you may need to replace the whole shaft. Ruined gear teeth on the shaft would also require replacing the whole shaft.

I have also seen a unit with messed up threads for the depth lock and quill lock knobs. That required a new shaft also.

The quill depth indicator is an area where I've found frequent factory assembly errors. The indicator notch in the spring housing needs to be straight up, aligned with the setscrew. If not tightened enough, the spring housing can slip out of position. Also, there are two serrated washers. On the inner one, the rough side faces out. Then the depth indicator goes on, being sure the stop will correctly indicate 0", then the second serrated washer goes on, rough side facing in. If you have a smooth washer, it goes on the shaft first.