Hi Tim
It sounds like you need to adjust the position of the bracket that is moved by the clutch release bearing and pulls on the interlock rod.
You need to set the gap “b” shown in the diagram. The words in the manual say this
“Depress clutch pedal to bring toggles JUST INTO CONTACTWITH THRUST RACE. Adjust gear lock withdrawal fork on bracket of thrust race in order to obtain a clearance ‘b’ *of 1 to 2 mm. between head of plunger and face of fork (see Drawing 6).”
But essentially you are making the clutch pull the interlock rod further than it does now.
Chris