Are you adding nitrogen? It could be either that it isn't getting enough nitrogen, or that you have it under too much light. I suggest moving it to a "shadier" place in your tank, if possible. Otherwise, cut back on your lighting or photoperiod. Java Fern needs almost no light to grow healthy, and under any lighting greater than 1.5 watts/gallon, it can wilt and die back. It won't completely die, but it will look burnt and unsightly.