The most common causes for a clutch to slip. Now you have to play Diick Tracey and find out the real cause.
1. Worn clutch disc. Disc too thin for proper pressure between the flywheel and pressure plate.
2. Weak pressure plate springs. Could be from overheating from too much slipping the clutch pedal or linkage adjustable.
3. Leaking slave cylinder. Fluid on disc and pressure plate.
4. Mis-adjusted linkage. Not allowing the slave cylinder to fully retract. And causing the unit to overheat.
5. Hot spots on the flywheel and pressure from riding the clutch pedal, and from the things listed above.
6. Mis-match parts.
7. Driver error. Riding the clutch pedal. The hardest one to prove. Everyone will denied it.
These are the most common causes that I have run into over the years.