It's probably natural that some will be born dead. I've never been so observant to be able to tell how many or what percentage were born dead. I just notice the ones that are swimming around and try to make sure they have good cover and receive appropriate food. As with humans, some will have more problems with pregnancy. If all of your guppies came from the same line, it could be an inherited trait. Guppies are pretty tough; however, they have their limitations. If the water is too soft and acid, that could certainly add to the issues. Get a test kit that gives readings for ammonia, nitrites, nitrates, PH, GH, and KH. By doing the tests and then checking the guppy profile here on the site, you might be able to pinpoint the problem.