Have a look in your parts book at the kick starter mechanism, you'll see its quite a complex assembly with all manner of springs clips, spacers and a gear
You're simply going to have to get that cover off and It's a good bet you'll see what the problem is
It's easier to work on if you rempve the clutch...but not strictly necessary, it can be removed and re-fitted with the clutch in place but it's awkward that way...saves a clutch tab washer though
at the end of the axle, you'll see it has a 'foot', check this comes to rest on a cast protrusion on the engine case when in the relaxed position.
also check the spring end is in the correct place in the hole next to the outer casing screw hole (see pic)