It's hard to say exactly why your creeping phlox are dying, but I've always found that these plants really require quick drainage. I had them die several times on me until I planted them on an east-facing slope where they get full sun most of the day, but the soil does not stay muddy due to the sloping conditions. Too much water can be a problem for these plants and I would not fertilize them if they are struggling. Sometimes the worst thing you can do to a sick plant is try to feed it. It's like force-feeding a sick plant.