Where are you keeping them? If the temperatures drop too low, the babies can freeze to death since they have no fur and ASFs aren't particularly good at making nests. They thrive at higher temperatures, since their natural home is Africa. I've found that anything below 70 or so degrees Fahrenheit is too cold for them to survive as pups, and thrive as adults. If you need to keep their temps up, since you have a tank, you can put a heat pad under one side on low. Make sure to monitor temps so it doesn't get too hot, but it can help.

The other thing that may have happened is that since you have a first time mom, she may have forgotten or didn't know how to feed them. This will likely be remedied by the second litter, but it is no less frustrating.