The only time the pencil comes off prematurely is when there are parts of the stone that are close to being flat and the built up slurry takes it off. This does take a bit though, the slurry doesn't take it off that fast.

I re-do the grid when all the pencil is removed and rub it a few more times. The pencil on the flat part comes right off and I can now see if there are still pencil marks. I have one coti that I lapped a few times and there is still pencil on one of the corners. (It's a bout and the corner is out of the razors way and would require too much material to be removed to get flat.)