I agree with the Quail Lady with possibilities on the feed. And yes, it could be 1500 other things, however feed does play a huge part in the growth of birds and have direct effects on health and can even lead to death.
I recently had an issue with one of my chickens that had a chronic sour crop which was going to lead her to her death if I did not figure out how to cure her. After vet meds, home remedies, 7 weeks and nearly dying, I finally changed her feed to another brand. INSTANT healing. And the culprit feed was a big name brand...Purina Layena.
So just because millions and millions of birds are being raised on a good diet of GBS, that does not mean that a few birds are not affected by that particular feed. Some birds just can not digest all the nutrients out of feed and may have health issues because of it, and not all feed is created equal. Some feeds are not all they claim to be and do lack in certain things. If the feed is not digestable enough, then nutrients are lost in the waste.
So if all else fails and you have tried every other avenue, try changing feeds. It could save a birds life.
And I do agree on the Vitamin E addition. Babies often get balance and neurological issues because of a lack of Vit E. Add selenium with the Vit E, helps the E become more usable in the body. They also boost the immune system. Good luck with your baby!