Yep This ^^^If it's shot, then it's shot.
But sometimes some people think it's shot when it isn't.
Maybe try cleaning it with a very stiff tooth brush and scouring powder.
I lap a lot quite often, I have to clean my plates off regularly or they slow down. I use Barkeepers Friend powder, you prob don't have that available but I am sure there is an equivalent.
I also have a very old 400x that is very tired. I don't use it for making slurry, last thing I want is a diamond nub popping off into my finishing slurry. Yes, it happens. If I want slurry, I'll use a slurry stone.
But - for final finish lapping of just about any stone - the beat up 400 Atoma works great.