4 Week old kittens won't poop but will pee

My sister, four days ago, found two four-week-old kittens behind a shed. The mother was around but the kittens were really sick and were in worrisome condition. She knew it wasn’t the best idea to take them from their mother, but because of the shape they were in she proceeded to do so anyways. She didn’t have time to take them(shes a mother of two) and therefore asked me to do it. I, of course, was happy to do so. They smelt really badly of urine, and they’re eyes had that goopiness to them, to where they couldn’t open them up. After I bathed and blow dried them, I cleaned their eyes so they could open them up. I tried feeding them, the older one, Lucy, ate really well; if anything she was really aggressive while eating. The other one, Frenchfry, is a runt and didn’t want to eat. He kept refusing to eat, so we got worried because of that, as well as the health issues they were showing; sneezing constantly, eyes contiously glazing over no matter how many times we wiped the goop away, and we could feel the liquid move around in their little lungs. On the second day I had them, my sister had made a vet appointment for them. We only took one to reduce the costs, in this case it was Frenchfry who was taken because he seemed the sickest of the two. The vet had given him deworming, and amoxicillin. He also said that there were in fact repository problems and they could have one of two different sicknesses(I apologize, I forgot what the names where). He said Frenchfry may not be eating due to sore throat, or because he doesn’t like the taste of the formula. So he recommended adding a bit of pancake batter, or honey. When I got home, I had neither of those things, so I figure maple syrup would suffice. And thankfully, frenchfry began taking the formula. It’s been three days since the vet visit, and we’ve been feeding them formula. They’re stomachs are pretty big, and pretty hard. It’s starting to worry me because Lucy hasn’t pooped in three days, and the last time frenchdry pooped was actually diarrhea before he went to the vet. The vet also recommended giving them them amoxicillin twice a day. I keep trying to rub they’re tummies and constantly stimulate their parts as well as butt; but to no avail. They pee just fine(though lately they haven’t peed as much as before) but they WILL NOT poop. Both of them won’t. I’m worried and I don’t want them to die because of it, they don’t overly act distressed or anything, if anything Lucy actually plays a lot, and sometimes frenchfry as well. Is syrup a bad thing to substitute for pancake mix or honey? Or is it the medicine, a side effect, the dewormer? Someone please help.