First, we need to know what the three feeds are. At her age (8 weeks) she should be just starting on 'grower', or staying on starter/grower feed. If she's been eating a good bit of grain, her diet has likely been deficient in a number of things including absorbed calcium, protein, and important vitamins for growth.
Particularly when you're relatively new to poultry, the babies should always have 'starter' crumbles as 95% of their diet for the first 6-8 weeks. The next 6-18 weeks should be a grower feed (one designed for the type of birds they're to be - broiler grower for slaughter, regular grower for laying or breeding or pets). At 18 weeks, you start to watch the combs of the pullets (which are still tight, pale, and dry). When they start to enlarge and redden, then you slowly integrate laying crumbles into the feed. That way they start to get their calcium, don't get too rapid a diet change, but don't come off of grower too fast as obviously they're still growing.
At about that same time, they should start to get calcium supplementation in the form of oyster shell. (Not egg shells, please.) That way the pullets who need a little more calcium than provided in the feed can self-supplement.
SO back to your baby. I'd make sure her feed is a starter/grower or grower. I would put her up in a smaller area and make sure she eats her feed and drinks water. Using a vitamin/mineral/electrolyte supplement for poultry (the silver packages from the feed store) will help her get back into shape. If she's not eating, I would make a damp mash out of crumbles, yogurt, and a little water for her first meal of the day.
The yogurt will help provide nutrients, but most importantly will help provide more living bacteria for her digestive tract. Poultry depend on living bacteria in their intestines, mostly lactobacilli but there are others as well. Those bacteria help to keep the birds healthier but also help them to absorb more nutrients from the food, use their food more efficiently.
Also if these babies aren't in a pen, or have any access to anything other than crumbles, they should be getting chick grit by this age. Chick grit pieces are smaller than normal chicken grit, but just as important. Birds who receive grit are shown to digest their food more efficiently and gain more weight. So you want to provide that in a separate container for them. When they get older, you can put the oyster shell in the same container. They're both important.
Since you have her up, you should take the time to check her carefully. Feel her weight - you should be able to feel her keel bone but there should be meat to either side of it. it shouldn't feel like a spatula. I suspect the change of feed, etc, might have caused her to stop eating. Also check her thoroughly for poultry lice or mites. Look particularly in warm moist places like under the wings, around the vent, and at the back of the neck. Check the feathers' bases for clusters of white eggs, and check the skin for redness or irritation.
Then check her breathing - both with your hand and with your ears. Listen to her breathing around her head, throat, and her chest. Put your hand there as sometimes you can 'feel' rattles more readily than hearing them. Examine her eyes, nares (nostrils), etc for any abnormalities.
Watch and report to us what her droppings are like (another reason to keep her up, that and making her feel more safe so she's ore likely to eat without competition or stress.) Droppings can tell you a lot about what's going on inside your bird.
These are things you should do to all of your birds at least once a month not only so that you become very familiar with them, but also so that you can catch problems ahead of time.
When you put her up, check her feeders/waterers/area carefully for any sign of mildew, mold, stagnant water, etc. If there are droppings or "biofilm" on the waterers or along the insides, clean them with soap and hot water, rinse well, and refill.
If she switched feed, make sure there's not something else she could have tried to eat if she didn't like the new food.
Also while you have her up, you can use boiled and mashed eggs to tempt her to eat and help her gain weight. A damp mash made by mixing yogurt (1t) with water (3 T's), a little boiled egg, and some crumbles - mixed and left to absorb for 10 minutes, can be a great health booster for this bird. It would not hurt, given her current state, to also give her some PolyViSol baby vitamins (not the iron fortified, but the non-iron-fortified) 3 drops in the beak for a week because of her weakness and reluctancy to move.
So for now:
Make sure she's on a proper feed: grower or starter grower, and is eating it.
Give her a daily supplement of yogurt, egg, vitamins for a week.
Keep her up until she's quite vigorous again.
Supplement all the other birds with a vitamin/mineral/electrolyte supplement.
Check to make sure the new food is very fresh.
Check the premises for any mildew/molds, etc.
Reduce grains to less than 10% of her total diet.
Please do let us know how she's doing. If we can do anything further to help, please feel free to ask. Thank you!
p.s. That includes asking for any more care tips for the future. Welcome to chickens!