A prime candidate for this could be:
- Worn Friction Plates
- I've not seen this personally
- Worn Steel Plates
- I've not seen this personally
- Incorrect Torque Settings to Clutch Springs
- Worn Clutch Springs
- Incorrect Gearbox Oil
- I have had this personally.
- Incorrect Assembly of Friction Plates
- The smallest friction plate should be installed first, the rest in any order.
You just want to use 10W-40 Semi-Synthetic Engine Oil, avoid Castrol Magnetec as I had clutch slipping issues using that type of oil.
If you've addressed the above issues and are still having problems then given the cheapness of a clutch rebuild kit I'd just grab EBC Dirt Bike clutch plates and new uprated springs and throw it back together.