You possibly have fostered longer than I have, but I thought at least I can share a few things I have learned.
Depending on how healthy they are, how often are you feeding them and how many ml’s are they eating at a time? I personally would be feeding kittens that age about every 2-3 hours, and would hope they are eating 2-3 ml’s, especially if they aren’t in perfect health. Once they get stronger, then I go about 6 hours overnight, 3-4 during the day for feedings. I also use an oral syringe (they have them at Amazon, possibly your pharmacy or your vet) with a nipple on the end. It’s sometimes easier to feed them with the oral syringe when they are very small, you can keep the flow going at a pace that you want and it’s easy to keep track of how much they are getting. (I could send you some if you want).
I also start out with goat’s milk, it just seems so much easier on their stomachs. My local Wal-Mart even carries it and it lasts a long time. Once the kittens are over the hump I start mixing in kitten formula. I do not use KMR, I personally think it’s not good. My kittens have always ended up with the runs after using that. I use a kitten powdered formula that I send for that you can get from Amazon or Revival Animal Health. Either way you order it, it still comes from Revival. Regular shipping is pretty fast, although it comes from Iowa and I am in Minnesota….. This formula really seems to sit well with all the bottle babies I have fostered. They poop pretty regular with this kitten formula as well.
The only problem I have had with bottle babies (the healthy ones, that is) is their need to suck. I decided just this morning that I am going to try to make a kitten pacifier. Kitten bottles are less than $1 a piece at the Wal-Mart close to me. Will see how it works – the poor things just have that desire for a mom….
It sounds like you are doing just about everything that I do and hopefully they are getting enough at a feeding to satisfy them.
Good luck, anxious to hear how they are doing.