Chicken not responding to meds

Hi everyone! I have a chicken (approx 4 yrs old) that started to show signs of illness: slow, not keeping up with the group. After examining the flock's stool I saw a couple of worms (cannot say if the wormy stool was hers or not) so treated the whole flock. After treatment she started with very runny stool...stool stuck to her feathers, et. So I treated them again approx. 2 weeks after the first treatment (diatomaceous earth). She would rebound a little but ultimately go back to lethargic, drinking a lot (due to dehydration from the runny stool I'm assuming), so took her stool in to vet. She was diagnosed with Coccidia was treated by vet for 5 days. She again rebounded in attitude but the runny stools continued, and I'm talking watery stool..no shape/consistency to it. She has lost a lot of weight as well by this point. So after waiting another 2 weeks after her vet treatment I went ahead and treated the whole flock with Corrid (7 days). She again rebounded but still continued with the runny stool. It is now 7 days after the Corrid treatment and she is back to being slow, quiet, not really pecking around like a chicken would. So....anyone else have an issue like this where a chicken only rebounds for a few days but then ultimately reverts back to not well and continues to have very runny stools (we are talking about 2 months of this now). I have drenched her, added electrolytes to water and also include yogurt off and on through out this time period. Diet hasn't changed, coop is clean, they have a very large yard and are also let out daily on to our 3+ acres (live on 60 acres but they only go around the house). Oh...she also continued to lay pretty consistently the whole time until this past week or so.