I will assume you tried a file card and short bristled stainless
steel brush. It is time consuming, but I use brass to slide between
the teeth. Most of the time I use rifle brass, but pistol brass works
ok also. Brazing rod might be more available for you unless you shoot
or know some one who frequents a shooting range, you can find brass
all over the ground usually. Place the open mouth at an angle to the
tooth of the file and slide parallel to the cut. It will take a few
times across to cut the pattern into the brass, but once it is there
it works pretty good. It won’t get all the way to the bottom of the
tooth, but it will recover a dirty file even with metal packed in,
also works on rust.
This is assuming that cleaning is the problem and not dulling of the
teeth. Anyhow, that’s my $0.02.