I'm so sorry... I know how you feel... it's so frustrating and sad losing a baby...
Since you're doing everything right and there's nothing apparently wrong w/ the puppies, I can only think it was a condition called "fadding puppy"... but it doesn't happen in all babies of the litter nor it's something contagious... it just happens without reasons...
Keep taking care of the ones left... my heart goes out to you...
Again... I'm soo sorry...
Katiuscia & Maggie...