(Click on the pictures to enlarge)
Check that the selector spring is in good state and fitted in the right sense (common mistake).
Correct sense for the spring.
Check that the washers and the o-ring are fitted correctly (spring-washer-oring-washer).
Fit temporarily the selector, select the gearbox in 4th (not neutral as advised in some manuals) and check that the travel is the same on top and bottom.
If necessary, adjust with the excentric screw.
Fitting a o-ring (non-original part) on the selector case helps to eliminate any oil leak if the axle is used.