Black marks on Primary clutch

Yep you are slipping in your primary. There is many different ways to approach this. Add weight to the heel or get weights that are the same as yours just a different profile so there is more weight in the heel. Or change to a lower finishing rate spring. Some of the danger of doing this is that it might mess with your RPM. If it is on one side then you need to check alignment.
It could also be that if you are on a 2 year old belt it is just wore out.

If you are ever moving fast or turning high track speed and hit the brakes it will do it, If you have good luck with belts, check your clutching and motor mounts, if they all check out then clean them and forget about it.