I understand that the standard blood word may not show the problem and therefore come back fine. A case in point being that of liver function. It will show up on standard blood work but only when the liver function has been reduced to the point just short of total liver failure and can't be treated effectively. It takes a bile acids test to detect the problem while it can be treated effectively. So unless the blood work is testing for vitamin deficiency, I am not certain that it would show up. All I know for sure is that this type of problem is normally either a dietary issue or a genetic one.
Honestly, this is all I know, and I am sharing it. What to do if anything is entirely in your hands and I don't know what else to do or if there is anything that can be done for certain. Just trying to express my concern for your bird and wishing you all the best.