There could be a couple culprits here.
First of all the clutch might not be adjusted correctly so when you pull the clutch in, there is still a load/pressure on the gears so when you go to kick up or down you essentially forcing the gear to slip from one to another. To fix this, lube the clutch cable and the lever then play try different adjustments on the clutch to see if the issue still occurs when up/down shifting.
It could also be a really worn out clutch, I had a KZ550 and it had the same issue I found out that my clutch was so old and worn out that instead of the clutch disks slipping they where sticking instead. The fix here is get a new set of clutch discs, might as well get a new clutch basket and springs.