Need Immediate Help Please! My cat had 7 Kittens and 2 Have Died

My cat had 7 kittens 5 days ago. She was at the vet 2 days before she gave birth as I actually thought she might be overdue and thankfully she was not but the vet commented that she didn't seem to have a lot of milk. I immediately upped her kitten food intake but didn't like the science diet or pro plan so the vet said to feed her whatever she wanted to eat,which turned out to be fancy feast appetizers. She said at least 4 of those per day. My cat gave birth Feb 13 and did not like the nest I had set up for her in the closet. Instead she had the kittens all over my room and she did disconnect the placentas from herself and the kittens but she didn't eat the placentas and left each one except one attached to each kitten. I was instructed by my vet how to handle cutting off the placentas. After her labor was over she was fine and I had all the kittens on a heating pad and she seemed to know what to do. On day 2 of the kitten's lives I noticed kitten number 2 who was born was very weak and lethargic and she wasn't eating and not gaining. I am weighing the kitties daily in kilograms. Not sure if that makes a difference or not? But this baby died that day? I tried everything. The next day kitten number 1 who was born first lost weight and got weak and cold and didn't want to eat. I got kmr and did like the other kitten. Took him to the vet and they gave him sub q fluids but he died a couple hours later. They are at mom and suckling and they gain one day and then they lose the next. I am not understanding this? The remaining 5 ALL gained great yesterday. I was thrilled. Today, 1 lost  3 kgs and one stayed the same and 3 gained. I immediately started feeding them kmr. Mom is eating like a pig. They are at her nipples and doing great.WHY is this happening? They are still acting ok and everything but what is going on? I don't want to lose anymore? Thanks for any help.

Them going down, may be several different reasons... At some we cant really do much. It may for example be pneumonia if they got water in the lungs...  The most common cause when healthy born kittens dies off.  There are several possibilities.

What we can do, is to cover the cases we can do help them.

Keeping them warm, keeping them hydrated. Cold or dehydrated, they lose strengh, and dont eat...

Keep Warm (but not too warm either), you can try to massage them some.

You can try and boost up the weak ones by letting on some white caro syrup on their gums. Dextrose/glucose sugar is of course also OK. Honey too if you dont have any of these...

Good luck!

Be very careful feeding them.  They must be on their tummies with their head up a little, just like when they feed from their mother, and don't feed them too fast.  Kittens of this age don't have a cough reflex and it's awful easy for it to go down the wrong way which will lead to a fatal pneumonia.