Some questions, and please understand trying to help....
Have you cleaned your bird's cage thoroughly... I mean take it out and steam clean it, or spray down with F10 (or bleach if F10 not available), let it dry in the sun and then scrub it thoroughly with a brush getting all the food and gunk out of the nooks and crannies and crevices. Literally clean this cage til it looks brand new, not a speck of food or poo anywhere... Very important.
Throw out the food you have been feeding and get new supplies. If your bird is constantly becoming reinfected, your food may be bad or contaminated.... depending on the food or how stored, if any dried bird poo gets into the container or storage, the food can be the ongoing reintroduction source.
Are you treating with Baycox in the water? Are you doing the usual 2 day treatment? Has your vet considered a secondary infection that is allowing the cycle to repeat.
But more than anything, please do a thorough deep cleaning of everything in the environment. The eggs in the droppings left on playstands, perches, sides of food bowls, etc. can be a problem. I do not want to suggest your husbandry is not excellent, but take additional steps over and beyond whatever you are doing... clean the cage every day. Wash the food and water bowls daily with soap and hot water. Wipe down the cages of the bars even after steam cleaning with a hot rag, be sure to clean up all poop / droppings in the cage, on perches, playstands, anywhere that your bird hangs out.
Good luck and I hope you are able to get the cycle stopped!