| The most probable cause is the clutch is dirty, full of dust. Take it apart, clean it out and the plates with brake cleaner and reassemble. It's not difficult. 30 minutes tops.
The only other possiblity is the dished drive plate in the clutch has flattened off. It'll be towards the back of the pack, and is denoted by a dot punch in it's face near the cutout in it's periphery.
Lay it on a flat surface and check if it's still dished. If it's anywhere near being flat, replace it with a new one for a few £'s. | 
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