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Well she isn't a really heavy breed, so I wouldn't think it's the type of problem you see in cornish x's for example. One thing that you're probably already doing but I'd be amiss if I didn't mention it: make sure that they all have grit, chick sized grit, no matter if there seem to be a lot of pebbles in the area. That really helps the digesti tract keep up to speed and has been shown to increase healthy weight gain.
Well one other thing - did you happen to check her crop? I doubt it's a crop issue, but...
Really I'd just watch her carefully for respiratory symptoms. Also check her throat (her gullet by opening her mouth) every couple of days while this is happening to see if you can see any reddening, white nodules, etc - compared to the other birds her age. I wonder if she just doesn't have something physically congenitally wrong with her.
Otherwise, I'd do the usual scattershot type treatment that I do for any bird that makes me wonder - give vitamins in the water a few times in the week, watch them, etc. Maybe put a couple of drops of VetRx down that one's throat. It's not medicinal but it does seem to straighten out some issues where there's no real good clear explaination yet.