Hi,
It’s probably down to wear and tear in the positive stop mechanism. When you operate the pedal does the indent ball engage with the relevant hole in the external plate on the front of the gearbox, or does it end up in between two gear positions? Before dismantling the positive stop mechanism (little springs and balls ready to go into orbit!) try adjusting the length of the short rod that connects the positive stop mechanism to the bellcrank arm.
Another thing to check is to take off the oil filler cap, and check that when in the second (middle) gear position on the external indent plate the gear selector dog is flush with the face of the second gear dogs. If that is correct the other gears should automatically line up when engaged, unless excess wear in the positive stop mechanism is preventing adequate movement of the selector drum.
Good luck!
Brian
