Urgent Chronically ill cockatiel... how to help?

[SamsonNZ]

One of my cockatiels goes through bouts of recurrent sickness.
These occur every few months (3-8 months). Symptoms include sneezing, open mouth breathing, and vomiting whole undigested seeds (not regurgitation).
None of my other birds are (nor have ever been) affected with these same symptoms. They are all kept in the same room.
Her cage is kept clean, her water and food changed out every day.

I've taken her to the vet each time she's fallen ill (around 6 times now?). She's been given antibiotics each time, which do seem to work, but it always comes back.
She was on her last round of antibiotics for three weeks and has been well for just under two months before falling sick again.

I'm frustrated and sad to see my wee bird suffering. The vet can't seem to pinpoint a diagnosis nor cause for her condition, and I can't keep paying through the nose for avian antibiotics that don't fix her issue.

Does anyone have any ideas about what might be causing this? Or ways that could potentially lessen her symptoms? Thank you.

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More info:
She is 6 years old and very likely the 'runt' of her clutch.
Unknown parentage, likely came from a backyard breeder (brought her before I knew better, unfortunately).
She's always been very quiet, mellow and slow. I do suspect that her immune system/function is poor.
It's currently winter where I live, so she hasn't been outside to be exposed to any disease from wild birds.

 

[expressmailtome]

Bump.

 

[sunnysmom]

My first cockatiel would get bacterial infections every few months, despite my best efforts to keep everything clean, etc. My vet determined that Sunny, my tiel, was not producing enough good bacteria- so basically had a low immune system. Sunny would get sick. He'd get antibiotics and probiotics and then about 2 months after the probiotics stopped, he would get sick again. Eventually, my vet and I decided to just keep him on probiotics and it made a huge difference. Instead of being sick 3-4 times a year, he was only sick maybe once every 2-3 years. I would discuss this possibility with your vet perhaps. Typically, birds aren't supposed to stay on probiotics but we decided in Sunny's case, it was the best course of action. And if a bird does get sick, they still need antibiotics. Probiotics isn't a "cure". I hope you get things figured out with your tiel.