I am not calling you a liar but how certain are you it is parvo? There are serveral things that puppies can get that can kill them/make them sick that isn't parvo. Cochcidiosis looks alot like parvo and can kill a pup if they get dehydrated enough. Worms can kill puppies as well even if you are worming them. I know a whole litter that was nearly lost due to worms and the pups were barely 6 weeks old. Just wondering if you had the vet do a necropsy or if you are just assuming parvo.
That happened to me awhile back. The wormer i was using didnt work. Dang WAL-MART!!

They showed alot of the symtoms of Parvo, but it was actually from the worms.
Bloody stool, wont eat/drink, wont hardly move/ wont hold their head up.
The only pups that lived were the ones that i sold, and got a good dose of a better wormer.
I thought it was parvo, but my aunt is a vet, and she is the one who checked them and told me what it was.
By that time i had already lost nearly every pup.