Tried lapping my new 4k/8k on 400 grit wet/dry sandpaper and my bathroom granite countertop.

I got about 90 % flat and smooth (confirmed with pencil grid) on both sides after a fair bit of work, but for some reason some of the corners and very edges on both the 4k and 8k won't go flat no matter what I do.

I'm about to cave and just buy a DMT.