Clean the transport gears and the mounting post with isopropyl alcohol. Do not lubricate.

There is a detent on the bottom of the lower transport gear that is held by a latch when the shutter is cocked.

Pull the latch out and push the lower transport gear all the way down so it can properly catch the detent.

Install the upper transport gear such that the gap on the right side of the latch is as small as possible without touching.