ASFs keep dying

[Malum Argenteum]

" I saw that one ASFs was on the brink of death and it died the same day"

Sounds like something unrelated to your husbandry. I used to sell live feeder mice to a pet store, and the store had one other regular breeder. I grew mine out a bit before selling them, but the other breeder did not, and his looked "weak" and dropped in the way you describe. Mice weaned just a little too early do poorly, and I presume ASFs do too.

I used to breed a few ASFs. It was long enough ago, and not such a positive experience, that I don't recall many details. I kept pairs in 10g fish tanks and fed them dog food (I've improved my feeder rodent care by now -- tubs and lab food -- but I don't mind admitting past shortcomings), and they did just as well as mice or rats. Kind of nasty critters, but that's not related to your current issue.

Without a slew of details, it would be hard for anyone so say conclusively what's going on. Too hot/cold, ventilation issues, moldy food, animals mishandled in transit, cages too small -- anything, really. There is usually more than one way of keeping any animal species well, but a nearly infinite number of things that can be the cause of problems. Perhaps the breeder you got them from would be the best person to send all the info that could possibly be relevant, and photos of everything, to see if they have any input.