Sorry you are having to go through this but your first course of action should be getting a heat lamp to hang over the box and then the next would be to make sure that the big fat healthy ones eat last. The 5 that are not gaining may be due to the bigger ones sucking most of the milk out, i watched it happen with a couple of bigs ones i had, it doesnt take them long to drain most of it and move onto another nipple and if the other ones are smaller they are easily kicked off a tit and probably ending up with an empty one. Also if the pups are not getting enough warm milk they get cold on the inside as well. I know its hard with a fading pup but sometimes you can do everything to help the pup and it still dies, dont beat yourself up about it, just know that you did everything you possibly could .
Another thing you could try would be to tube feed the little one that is not sucking, i would get your vet to show you how to do it and how much formula to give. It could be the life saver for the little one .
But number 1 thing you need to get is a heat lamp, because when your at work how do you know the mom is laying with them and keeping them warm, thats the main killer of pups, they can not regulate their own body temperature for the first 2 weeks of life, they do not have the shiver reflex yet