Another thought is bacterial secondary infection from the coccidia and or capillary worms.  They can sometimes badly disrupt the gut bacteria, allowing the bad bacteria to take over.  E coli and clostridium are common and they can develop necrotic enteritis.  So a round of  antibiotics might be needed.  I've had secondary bacterial issues with roundworm infection, if they don't bounce back after worming I usually try amoxicillin first (if you have none on hand you may not be able to get it right now due to shortages- it's available as a fish medication without prescription).  Your vet that did the fecal could also check for bacteria.  Without amoxicillin I would try SMZ-TMP instead, you can get that here without prescription:  
https://www.medi-vet.com/Bird-Sulfa-Sulfamethoxazole-Trimethoprim-480mg-p/19328.htm
Worth a shot anyway, to see if it helps. 
Following the antibiotic I would give a good probiotic for a couple of weeks, to help the gut get back in good balance again. 
If you lose this one, then I would also highly recommend necropsy, so that you know exactly what is going on with them, especially since you've lost several.