I cant better the advice given already, I'm only chiming in because your coticule looks nearly identical to mine which seems to have the same bother of being able to shave very well but, not giving smoothness as much.
the only difference being mines seems very hard - not soft like yours - but strangely chalky to hone on like your coticule.
The only thing I can advise is slightly slow down your strokes and lighten them up a bit, my theory is the chalkyness indicates alot more particles hitting the edge than a smoother silky slurry, IMO creating a rougher edge. also does your coticule seem to slow down and hold back the speed of your strokes? sort of clinging onto the razor?
I can set bevels with little pressure after dulling on glass with the coticule I have. It seems the slurry cuts quick.
I would advise when finishing to try finishing on one half of the stone and then the other half to see if there is a discernible difference in edge.
I have found a marked improvement finishing on one half as opposed to the other.
I have an extremely hard la verte and bbw, both have very little inclusions and I have no problems getting keen smooth edges from either, but this coticule still gives me trouble sometimes, even after 2 years, I'm experimenting with it to the extremes now; very minute details like speed of stroke and subtle pressure differences and where on the coticule to hone... yes coticule honing has driven me nuts ;)
I wish you luck with your coticule and I will keep on at mine and hopefully find a technique to work with these sometimes stubborn rocks...
regards Alex