i'm not familiar with your bike.. Since your external parts are OK, i guess you need to look a little deeper.
i'm going from memory about a bike i had with a similar clutch/lever starting setup. But my bike was a two speed and the clutch was in with the tranny gears and was wet.
The external lever turned a shaft which goes down through the case.. That shaft had a fork on the end which moved another lever that engages the clutch. On my bike, this internal fork/lever setup had another spring. All this stuff was accessed by removing a side cover.
Hopefully as you work your way in you will see a mechanical linkage problem. If not, the clutch pads or springs may be at fault.