Sounds like either you have a weaker strain of blackhead, or stronger birds with some natural immunity, or, worst case scenario, you have a chronic long-lasting strain of blackhead. They can relapse indefinitely.
If they're getting less yellow poops all on their own, chances are they're naturally immune to some degree. They may handle it themselves. In my experience, direct siblings from the same upbringing can include some which die, some which never show symptoms, and some which get mildly sick then recover unaided. Individual strengths and weaknesses.
Isolating the bird is useless as a preventative for blackhead. They most common method of transmission is via soil contamination. The only reason to isolate her would be to make sure she receives her medication without the others taking it, but it seems this isn't going to be necessary in this case. Still, even if one seems sick and the rest seem healthy, they've all been exposed and you may run into a weak case or two in future. Might be best to cull those that are weak to it, judging by my experiences, since they can and do develop some kind of immunity against it, and it's fairly heritable.
I would lime all the soils around your place to help clean them of the contamination. Hydrated agricultural lime destroys many diseases, parasite eggs, and other issues.
Best wishes.