Do a big water change and see how the Java Fern goes.
It can be helpful to try different plants and see which ones do best in your particular tank. Ambulia, which you already have is normally a good plant for beginners. Hygrophila polysperma, Hydrilla and narrow Vallis are generally pretty easy to grow. Water Sprite is also good, I can't remember if you already have that.
If you want to try an Amazon sword plant you can put them in a 1 or 2 liter plastic container like an icecream bucket, put an inch of gravel on the bottom, then a thin layer of garden fertiliser (actual fertiliser for the garden outside). Cover that with a 6mm (1/4 inch) layer of red clay, then fill the pot with gravel. Plant the sword plant into that and let it go. The clay stops the fertiliser leaching into the water and when the plant roots hit the clay and fertiliser, the plant goes nuts. You can do the same with Cryptocorynes, although they are a bit more sensitive to being moved.