Check this link to Hitchcocks tech notes:
https://www.hitchcocksmotorcycles.com/pictures/content178/albion_gearbox_rebuild_part_4.pdfThis is an article on rebuilding the 4-speed gearbox. The ratchet mechanism is under the outer cover of gearbox. First check to see that the ratchet springs are not broken. If they are, replacement is necessary.
Part 4 has the replacement discussion. Photo 18 (below) shows the ratchet mechanism and stop plate. The stop plate can be adjusted loosening the shoulder bolts, moving the stop plate such that the ratchet moves to just the correct spot. You need to know if when you shift, the ratchet moves just past the correct gear or just before the correct gear. Adjust the stop plate appropriately. Then test, first on the bench, then put it back together for a street test. When doing this for the first time, you will likely have to open this up several times to get it just right. Be very patient, if the gear goes past the proper gear on the up shift , rotate the stop plate
counter clockwise ever so slightly (1/2 to 1 mm). If the shift on upshift does not get to the next gear, rotate stop plate ever so slightly
clockwise.
If on down shift, the gear does not get to the lower gear, rotate stop plate
counter clockwise ever so slightly (1/2-1 mm).
If on down shift, the gear passes the intended lower gear, rotate stop plate
clockwise ever so slightly (1/2-1 mm).
If this does not improve, next check the detent plunger (to be discussed later) it could be worn or spring broken, or rounded end, or not adjusted.