How much oil and what type do you have in the primary case?
Too much oil can cause some of the plates to stick together , so can
oil that is too thick.You only need the chain to be a few mil into the
oil and it will spread it around lightly without getting the clutch
too wet.
I changed to ATF but if you try this first you will have to degrease
the case and its contents as the two do not mix. Then make sure your
gasket sealant is ATF proof, I didn't! and after spending a afternoon
doing the change over I put the bike away for the night, Next morning
the ATF was ALL on the floor !
Good luck, hope it turns out to be something simple (and cheap!)

Martyn