Free spining sounds like the main shaft is dropping down allowing the roll pin to drop out of the bottom gear. Or the roll pin snapped which is unlikely.
On one of my older jacks, I had to add an extra washer below the main shaft roll bin to get it to maintain the correct height. This was was caused by a slightly bowed jack which required a decent force to raise it.
A jamming of the gears will be caused when the main gear drops down allowing the teeth to hit the teeth of the side gear instead of meshing with them. This is caused by the upper bearing being worn. A large flat washer under this bearing can help alleviate this issue.
Just remember when playing with making minor adjustments to correct wear, that the main shaft moves vertically a small amount as a load is put on the jack or you are trying to retract the jack.