cat not producing milk?

My cat had 1 kitten 5 days ago. When she first had the kitten she was milking up well with 4 of her bags being pretty full. Then they started to go flat, which I thought was because the kitten was drinking it all quickly. But now they don't seem to be milking up at all. Her stomach/bags are now flat. Completely flat and not bulging with milk. I'm not completely sure whether she's producing enough for the one baby she has or if her body has stopped making milk because she only had one baby? How can I make sure she is making enough milk? I'm worried the kitten will starve since it is so young.

I also have been feeding her dry kibble, wet food, lactose free milk and water.

I rescue lots of cats and help them and for the past couple weeks I was working with an inexperienced young mother cat and today I awoke to her kittens suddenly dying. The vet wasn't able to do jackshit and I was sitting around crying like a dumbass trying to warm them up because they were freezing even though there was no reason for them to. Everything was warm like it was supposed to be. The mom was trying to feed them but she's still a baby herself so she'd wander off and go play so I would end up formula feeding them with family help.

They were doing so well I don't know what happened. Their eyes were opening and they were beginning tl walk. I've worked with several different cats before and this is my first experience. When fostering kittens, I know that sometimes one or two don't make it but I've never seen a whole litter die before. When I had to remove the last kitten from the mom she was licking it and trying to get it to move but the little guy was gone. She doesn't understand and fuck I feel like I just failed her because my job was to help them when she couldn't since she couldn't fully understand how to mother when shes just barely a year old.

I love rescuing animals but this bit me right in the ass and now I'm trying to comfort the mom cat while being a blubbering baby myself. I'm not gonna give up saving cats but I just feel so defeated right now.