1. Listen to Bigspendur!
But in case you want my opinion / experience:
Sometimes this happens to me. Nothing to do with a coticule being too coarse. I have been able to isolate 2 reasons so far, addressing them solves the problem:
a) my coticule is smaller than my norton. I rarely do X on the norton, but do it on the coticule - sometimes doing the X i might accidentally dull the razor on the edge of the coticule (even though it's rounded) - back to the norton for a few to HHT, then coticule but more carefully
b) the slurry for the coticule is too thick (not much water) - this dull the edge like there's no tomorrow
I suspect there might be a 3rd reason: particles off the rubbing stone are too big and dull the edge sometimes (totally random)
Hope this helps
Cheers
Ivo