The vitamins in feed are in adequate amounts to support normal levels, but when a chick has a deficiency the supplement levels need to be higher in order to reverse the deficiency. Once the deficiency is reversed then normal levels in feed should maintain them again. The usual treatment follows supplementing to see if you can get some improvement, and very often it does. If it is an injury or illness then the supplementing is not likely to help and there is really not much else you can do. Time will tell in those cases. I would go ahead and supplement and hope for some improvement. Vitamin deficiencies are relatively common and can have some very odd neurological symptoms. Vitamins in feed degrade with time, so older feed can contribute. A chick that is lower in pecking order can be kept from feed and suffer deficiencies, deficiencies can start while the chick is developing in the egg before hatch based on the health of the parent.