I am so sorry for the loss of your babies. I had a little one who I hand fed on his mother, I would put them both on my bed and hold his head still or he would bop off, or mom would lick him off. He did not gain weight until the 5th day, but he did not lose any either. I had a postal scale I would weigh each pup every day. The babies will be 10 weeks tomorrow, and he is still much smaller, but he is fine. I also thought the length of breeding time (whenever they wanted, as they are my pets) was the cause. Sometimes it is just how it was meant to be, and there might have been nothing anyone could have done. You certainly put out the love and effort.
Welcome to YT - there are a lot of people here who can help you, I hope you will stick around and let us know how your mom and puppy are doing! Post pictures