If you have a well-established sword plant that is dominating the tank, it can be hard for any other plants to get started and grow except maybe java fern, which tolerates low light and low nutrients by slowing down its growth, but not dying. Vallisneria and Hygrophila polysperma do need some room, and some light, especially the Val. The old-fashioned big Amazon sword can dominate a tank, even when the light is pretty low, around 1 to 2 watts fluorescent per gallon. Still, it seems odd that the Val should die in two weeks.