As one of the of the previous posters has said its always best to consult a vet and have proper tests done there are such a wide range of Diseases and Infections that chickens are susceptible to.
There are a number of reasons I don't believe it to be Coccidiosis at least not now anyway...I don't have an exact idea of your timescales with regards to you getting your chickens then them getting Sick and how long exactly it has been since they where sick. There are many types of Coccidiosis 10 or more I think. The most common are Eimeria acervulina, Eimeria Maxima and Eimeria tenella with the latter being one of the most common and the type you are more than likely to have/had. Immunity to one type does not give immunity to the other.
The main reason I would say you possibly don't have cocci is That the life cycle of Coccidiosis is pretty short 4-10 days. First seen around 3 days after infection by day four some symptoms may be visible with some blood, Blood will be most visible on days 5 and 6 by day 7-10 the bird will either be dead or recovering. I don't know exactly how long your birds have been sick etc so I could be wrong!! They may have had it for sure but it can lead to other chronic diseases so this is what I would look at. In the industry they use products such Monteban, Maxiban and Elancoban in the feed as a preventative measure and for treatment which is extremely effective unfortunately these are not widely available to buy.
Your best bet would be to get tests done you could treat them forever and still not be treating the right thing.