Sad kitten stories

It's been a couple of rough days for my fostering. I have two nursing momma cats in my fostering room. Chika has two 3 week old kittens that are thriving. She was rescued after two of her original four kittens were killed by tomcats in her area at a North Central feeding station. Sammy is a young semi-feral tabby cat who had kittens shortly after she was trapped at my own feeding station in my backyard. The kittens are about 6 days old now. She had 5 kittens, but one white one was dead after birth. I think she might have lain on it. Sammy has really struggled in caring for her kittens, so I had to start trying to intervene two days ago. I had been trying to be hands off because I was worried that with her being semi-feral it might make her neglect the kittens, but she is a first time mom, and things were going downhill. She still had them in the corner where she had birthed them, with a bare concrete floor, and doesn't seem to be nursing well. This despite the fact that I have five other nests set up around the room for her - she just wouldn't move them. Yesterday she rejected the other white one, and I found it lying alone on the floor. I worked late into the night with a hot water bottle and a bottle of formula, but it died in my lap. I even let my dog Atti, who loves kittens, lick the kitten all over in an effort to stimulate it to nurse from the bottle. When I got home this afternoon the black one was dead on the floor. I think she might have lain on it too. So that just leaves the tabby and the orange, and the tabby isn't looking good. I moved them into a proper nest with a heating pad, and left them there for about half an hour, hoping that their cries would prompt Sammy to move in with them. It didn't. So, as a last ditch effort, I moved them into Chika's nest. Thank goodness for her. She rolled over and started nursing them, and was washing them too. Her two kittens don't seem to mind at all. I stayed for about an hour, monitoring them, and will go back and check on them again in a bit. Don't think I'll sleep much tonight.
-Sarah