Make sure the chain is at the correct tension.
pull back the rubber covers on the rose joint on the selector rods between the pedal and the gear shaft, clean and re-grease.
Remove the front sprocket cover, thoroughly de-mank the accumulated shite and crud inside then when clean re-grease the clutch mechanism (DO NOT BE TEMPTED TO PULL THE CLUTCH PUSH ROD ALL THE WAY OUT!)
Replace front sprocket cover, and rest the push mechanism under the rubber bung.
Then any fine tuning can be done at the lever end.
If the problem persists you have either done something wrong or its a problem further in the engine.....