Periodic Heavy Gearshifts - 2013 Hyperstrada 821 SP

Hi
I have an 821 SP from 2013 with around 31.000km on the clock.
I have installed a rearset that makes GP shift (reverse shift).
But the gearchange is sometimes very heavy (both up and down) - at other times (rarely) it feels like a hot knife in butter.
But mostly it is close til impossible to put it in neutral.
I’m pretty sure, that its not the rearset, that causes my problem.

Does anyone have any idea, what causes my problem?

I haven’t tried to reinstall the original rearset (yet).

Thanks for your answer.
My oil has been used for about 1500km, so I dont think that is the cause of my problem.

I will install the original rearset, and find out if that makes any difference.

I owe you guys an update on my bike.
After having tested everything I could think of (the angle of the shifterrod, lenght of the clutchcable, broken parts in the clutch, missing parts in the clutch etc.) I ended up changing the oil and filter.
Now the clutch works perfect and the gearshift is nice and smooth.
Apparently the old oil made the clutchplates sticky and prevented it from disengaging the clutch.

The old oil was topped up by an authorized Ducati workshop.
They only use Ducati-certified oil, that works with both dry and wet clutches.
I have driven approximately 2000km with the old oil.
The workshop is as confused as I am.