I am new here and low on the totem pole. However I am in the midst of going through this same problem with mine. I brought home 4 chicks, and it seems they had something (Coryza?). I put them on Sulmet, a sulfa antibiotic. It helped one, then one of the adults got it, .. Finally, some were well, some were not. But not responding to Sulmet means that I needed to try another drug. So, I added Tetracycline to the water. At last, my 2 gurgling hens seem to be gurgling LESS! It's been a long haul.
So, I suggest you try one then try another if you don't see an improvement soon. I decided not to do Tylan and inject into the breast. I added it to the water.
Good luck on your journey!