Hi-
I wish I knew why this forum so so quiet lately (or perhaps it was unusually active when I joined a while ago). I was going to leave this to others, as some of my posts have missed the mark, but I do have a hunch on this.
I suspect that your problem is not what you think it is. The lampblack serves to highlight other problems/impurities and it should be easy to tell if you have any left any on the plate. If you have removed all of the impurities after polishing and you can wipe the whole plate with ultrasuede and it comes up clean, then go ahead and put some lampblack on. You don’t need a ton, but perhaps more than you have been using. Then with a new piece of ultrasuede, distribute the lampblack over the plate. Keep using new pieces of ultrasuede to pick up the lampblack until they come up clean. At that point, you can be pretty sure that your plate has no contaminants on the surface. If you still have spots then it could have something to with issues with the plate or with impurities in the fixer, or perhaps the fixer is too strong.
As always, more details might help. Have you been reusing the same plates?