What is the temperature of the formula you are feeding them? And it is a baby bird handfeeding formula, correct?
So you are keeping them in a Brooder/homemade Brooder? It's best if they were being abused, she'll likely kill them if you leave them in with her, so they should all be pulled and put into a brooder...
Breeders don't typically pull baby birds from the nest-box to hand-raise/hand-feed until they are 2 weeks old...So the babies that you have are between 2-3 weeks old, they are extremely young and must be kept right around 95 degrees F ambient, and the formula cannot be outside of 104-110...When you make your feeding schedule, you need to go by their age in weeks...
I hate to tell you this, but they are not old enough to go overnight without eating yet...You need to be feeding them every 2-3 hours round the clock until they are 5 weeks old, at that point they can go overnight 6 hours without eating...But right now, you need to set a clock and wake up every 2 hours during the night until each one hits at the very least 4 weeks old, 5 is best, as that is what mom and dad would be doing...So this could very well be your problem, or at least a huge part of it (the formula temperature needs to be #1 priority)...
***So until they hit 3 weeks old you need to feed them once every 2 hours including overnight...At 3 weeks old it's once every 3 hours, including overnight...At 4 weeks old you need to feed them once every 3 hours as well, and should be overnight. At 5 weeks old you can feed them every 3-4 hours, based on the contents of their crops, and they can go 6-8 hours overnight...and at this point you should be giving them millet sprays inside the brooder...And so on...Sorry, but that's how it goes. If they aren't fed overnight up until at least 4 weeks old, they can become seriously malnourished and die, and this is most likely your issue...
Always check their crops before feeding them...I didn't realize they were this young when I wrote my first post or I would have asked you about feeding them overnight...Their crops should be almost empty each time you feed them, if they aren't almost empty then wait...Once they are at least 4 weeks old and they can go overnight 6 hours without a feeding, they should be emptying completely over that 6 hours...
Bottom line is that their crops cannot be empty at all until they hit 4 weeks old, 5 is better...All bird breeders that hand-raise gotta do it, just set an alarm clock to go off every 2 hours...Sorry, it's just a part of the gig...Another reason to not allow anymore eggs to hatch, lol...Also, if their crops ever slow down or if they hit the 4 week mark and you start letting them go 6 hours overnight without eating, and their crops are not completely empty in the morning, then this is usually a sign of a yeast infection, and there are lots of things you can do at that point...But right now I think your issue is that they haven't been being fed overnight at all, and possibly the formula temperature, depending on what you're doing with it...
And try to keep the formula on the thinner side, and then gradually thicken it as they get older. And as they get older, they'll usually start to refuse formula that is on the 104 degree F end and want it closer to the 110 degree F end...But i think if you get them all on the correct feeding schedule and amount, you'll be find...Oh, never stop feeding them based on a certain amount of formula!!! 2ml may be enough, it may not, the only way to tell if they've eaten enough is by feeling their crops! You cannot go by a chart or a certain amount that someone wrote online somewhere based on their age, that isn't how it works...They should be giving you a feeding response that will allow for their crops to become very large and round, like a balloon, but still have some give...you don't want the balloon to be hard, you want it to feel like a big, inflated balloon that still has a little bit of give, and then you stop feeding them, regardless of the amount (NEVER FORCE ANY FORMULA INTO THEM WITHOUT A FEEDING RESPONSE, NOT EVER!!! YOU'LL ASPIRATE THEM!!!). So it could also be that not only have they not been getting enough formula feedings, but the amount hasn't been enough if you're always stopping each one at 2ml. Each one will eat a totally different amount based on their age, size, crop size, body size, weight, etc. Oh, you need a digital kitchen scale if you don't already have one as they all need to be weighed once each day, first thing in the morning at the same time, and you need to write it down, as this is the only way to know if one is losing...
So I think they are probably a little malnourished and also undernourished, and depending on whether you've been using a cooking/candy thermometer in the formula at all times and making sure the formula is ALWAYS between 104-110, this isn't optional, then they may also have a bit of a yeast infection in their crops, but I don't know because you didn't mention the formula temperature...
Also, if you're using Harrison's formula and they have different types, and it's not giving good results, which it isn't, you're much better off, believe it or not, to run to your local Petco or Petsmart and buy a canister of plain old Kaytee Hand-Feeding Formula. It's great stuff, they love it, it contains Probiotics already added (I usually added my own anyway), and I never once had any nutritional or weigh issues when using it. Sometimes the more expensive, "fancier", for lack of a better word, hand-feeding formulas aren't the best for the babies...I used Roudybush for about 10 years and then the Kaytee for another 10 years, and both were great. You said something about a "neonate formula", which made me think you weren't even using a hand-feeding formula for baby birds...If that is the case, then that's a huge problem, and they'll die if you are using anything made for humans, puppies, kittens, etc. It must be a commercially prepared/sold hand-feeding formula for baby birds ONLY! People who live in other countries, usually in the middle-east where they can't find a commercial hand-feeding formula available, will use a human baby formula or cereal, usually Cerelac, which is loaded with iron (birds cannot process much iron, it kills them), and it also does not provide near the nutrition they need as baby birds...When in doubt, use Kaytee, it's easy to find, cheap, and it works perfectly...It's not uncommon for breeders, even experienced breeders, to have to switch formula brands because the one they are using is causing issues.
***Breeding/hand-raising/hand-feeding birds is not for the faint of heart, it's not easy, and if you don't know what you're doing you shouldn't do it...Most breeders actually do a mentorship/internship with an experienced breeder before ever even attempting...And I know you didn't want to hand-feed/hand-raise baby birds, but any time you allow eggs to hatch, even if you're expecting the parents to feed/raise the babies, look what can happen!!! Sorry about the feeding overnight thing, but it's not optional...