Hi brownysfp,
unfortunately there are many things that can cause a chick to fail to thrive. Runting-stunting syndrome is one illness where the bird doesn't grow properly (I gather it's caused by a combination of a virus and heat treated mealed feeds). Coccidiosis is another common cause of failure to thrive, and of course there could be a congenital gut problem (though that's probably rarer). An infection with e. coli could cause low growth rate, as could marek's. And of course mould toxins in feed could also cause it.
I guess what each of the above have in common is that they stop the digestive system working properly to absorb nutrients. For instance in runting-stunting syndrome, the gizzard is often eroded, and if looked at in autopsy, the join between gizzard and proventriculus (the widened tube just before the gizzard) is often slack and overly wide. All this means that food isn't passing properly through the digestive system, and/or it's not breaking down properly to release its nutrients. Then there may be intestinal lining irritation so that even if the food is being broken down, the nutrients aren't properly picked up.
Given all the above, it's difficult to suggest a treatment. Perhaps you could try giving probiotics (e.g. kefir) to try to improve digestive health. Or you could try antibiotics first (then probiotics) if you're that way inclined (in case it's e. coli). Perhaps you could use a combination of added vitamins, probiotics and a feed change (e.g. make up batches of fresh food ground in a blender so they're more easily absorbed). Unfortunately the fact remains that birds that don't grow properly as chicks usually never thrive.
I hope you get somewhere with this one, apologies for a long and rather downbeat post.
regards
Erica