I agree with the above. Those on the bottom of the pecking order will avoid the feed station simply because they will be pecked by the more dominant birds. They may sneak a bite now and then but mostly they will look for spilled feed on the ground, which causes it's own problems. Moldy feed, parasites, etc. More feeders or stations is helpful in this type of situation. Also, some may just not have adjusted from free range diet to confined diet. I don't know if it happens in chickens (although some of mine seem to be this way when confined with nothing but layer feed to eat), but I was told by my vet that parrots will starve themselves if you try to abruptly change their diets. They simply won't eat if they don't like it.
Mareks can also cause internal tumors and weight loss and death. So there is that angle also.