That's a good point Lawny. Using the wrong oil, or low quality oil could be causing problems too, conventional oils, especially cheap ones, have a tendance to leave deposits, these deposits are likely to build up on things like clutch plates and cause slipping. Automotive oil uses friction modifiers that are not good for wet clutch applications, that will also cause slipping. I would start by changing your oil, and while the oil is drained, take the clutch cover off, take the clutch apart and inspect the clutch components for damage, excessive wear, or deposits on the discs and plates. Make sure all the linkage is in proper order too.
Yours truly, sincerelly, love always, Josh aka The DirtDemon.