Welcome to CC! That problem could be caused by a number of things, I'd start simple and lube all of the shifter linkage, adjust the clutch pushrod and the cable, also adjust your primary chain. When you take the derby cover off to adjust the pushrod look into the bottom of the primary and make sure that there is only a 1/2" of oil
(or so) in the primary, if it's allot more than that you might have a scavenge problem in the primary and you"ll need to check your feed and return hoses. The next step would be to take the plates out of the basket and check for excessive wear.