In the past I have lost a budgie to AGY. I was fairly new to budgies so I did not recognize the symptoms early enough which mostly was head bobbing and vomiting. He was on quite a few different medications for 2 month with no effect at all. He entered into the "wasting" part of the disease process. The bird still eats but the nutrients are no longer absorbed and malnutrition sets in.
My male budgie Murphy (current) began to open and close his jaw often so off to the vet for gram stains. Sure enough he has yeast in crop. He started on Amphotericin B right away for 10 days and it eradicated the yeast.
I believe my birds got the yeast from eating fresh corn on the cob. This is my own theory so please don't think it is medical advice. We know each corn kernel has an outer shell which is not only difficult for humans to digest it is the same way for small birds. The shell may remain in the crop longer than necessary before passing to the gizzard. In some cases it can be the bird will remove the heart of the corn kernel before eating it and spitting out the outer shell. However, I did not notice any sort of outer shell after they ate corn. Could it be that the shell of the kernel, while in the crop, develops yeast? I discussed this with my avian vet and he thinks its a strong possibility. Since I stopped feeding fresh corn there has been no signs of AGY in my birds. I also give them ACV once per month for a few days in their water. If your bird has an early case of AGY you stand a good chance of getting rid of the yeast. Try not to feed any food with sugar or honey because yeast cells can thrive in a sugary environment.
Best of luck.