Just as others have suggested, I want to reiterate that the possibility of cocci, and I want to clarify for you that while some cocci strains are weakened by Sulmet, others are not, and Sulmet can be hard on the already weakened bird.
Amprolium, whether Corid or another brand, is a Thiamin blocker and weakens or kills many strains of cocci without doing any damage to the bird. It is found in different brands, and generally you will find it with cattle medicines in feed and farm stores. I urge you to try this to be thorough in attempting to treat this illness. You won't be sure you have really tried unless you do this, as Sulmet is only trying half the possibilities.
I wish you luck. Don't give up and be thorough in searching for the answer!