Only once had problem you describe and it was before fitting a very open clutch cover. Simply stripped out the plates which were dirty/dusty one even a slight surface rust!!

Cleaned with a little 1200 grit on the steels and eh presto fine again.

I now simply get the compressor and whilst the engine is running pull in the clutch and blow out all the s**e, never had the problem since I started doing that, maybe a coincidence?!?!?!

Slippers will get dirty quicker simply by the fact they are slipping if you are gearing down quickly, I find there is a lot of s**e blown out after a trackday working it hard.

Assume you are using sintered frictions??