I had this happen last year, about this time with chickens about the same age. Not coccidia because they were very active and had good ravenous appetites. Possibly worms, but when I wormed them there was no sign (no dead worms in the stool) and they did not show improvement. One started sneezing, so I treated the whole flock with Denagard (maybe mycoplasma?), but there was no improvement for a couple of weeks after that.
Still a mystery to me what the problem was. I had never medicated them at all and was hesitant about the idea, but the diarrhea was so terrible and they had had it for several weeks, so I was getting desperate. I was very concerned that, in their weakened state, they would come down with a secondary infection. Not sure if any of the meds actually helped or if it was just a matter of time. Maybe some odd virus they picked up?
All 11 pullets in the flock eventually came down with this to varying degrees, and all lost quite a bit of weight. Good news is they recovered and are now happy hens laying lots of eggs.
Wish I could be more help. Whatever you decide to try, I would suggest putting some vitamins in their water (sav-a-chick) and/or some probiotics.
Good luck and keep us updated.