I am having trouble figuring out why the LB itself would decline in performance. It seems to me that the construction is so simple that this wouldn't be an issue unless the cause was obvious. For example, I would have thought that if the screen is getting clogged, someone would have mentioned that. If it is separating from the copper-clad conducting rods I would have thought someone would have mentioned that too.
I noticed with my LBs that when I used to load all the way up to the shoulder of the trench, I got a buildup along the shoulder, could that be the cause of this decline? If so, they can be cleaned with ISO, careful brushing, and a couple of empty-trench heat cycles (I used 30-45 seconds).
Other than that, I'd blame the batteries, but both isis and champ3185 have said they tried other batteries. I find the whole thing puzzling because I can't come up with a theory, and I hate mysteries like this.