Had a similar problem, with my 20K SG when. I had lapped it but it was leaving a rough edge and stria pattern.

I re-gridded the stone and took one lap with a worn 600 plate and saw a slight 1X1.5 in depression in the middle and center of the hone.
I re-lapped it, several times, removing a bit of the face, and it is a hard stone… and that solved the problem.

Take another look at your stone. Grid it and take one lap on a high grit plate or wet & dry. If you have a depression as the stone wears the un-lapped portion comes to the surface.

Or it could just be a pressure issue…