In all likelihood you're not doing anything wrong. There is a syndrome that litters sometimes fall prey to called "Fading Puppy Syndrome", and it happens in the exact manner in which you are losing your puppies. If you really want to pursue the issue, you can take the dead puppies to a vet and have necropsies done on them to determine the cause of death, but if it's Fading Puppy Syndrome the tests will probably be inconclusive.
Have you checked to make sure the mother is producing milk? Sometines, even though the teats are swollen and LOOK like they have milk, the puppies are sucking like ganvbusters but are not getting enough nourishment. If she has enough milk, a gentle tug on one of her nipples will produce big fat drops of milk from the nipple. If you have to work hard to get milk from her, then chances are the puppies are having the same problem.
The litter could have puppy herpes. This sometimes wipes out entire litters and there's nothing that you could have really done to prevent it.
How are the puppies behaving? Are they quiet? Are they noisy? Watch them when they are sleeping. A healthy puppy will twitch and jerk in its sleep as the nervous system "wakes up". If there is no twitching taking place, something may be wrong.
I'm sorry you are going through this. There is nothing more frustrating than losing puppies that have been planned for and waited for. My condolences to you on their loss.
Ghilly