check the bolt on top of the gearbox, outer left in a row of 5: it is hollow, has a ball on the underside, loaded by a spring inside theis hollow bolt: It causes the "click" when you change the gears.
the shift fork pushrod might be out of adjustment as well: You can adjust its length when the gearbox lid is off, first put it in gear with the lever, then take the lid off and check if the gears are fully meshed in this position.