Ammania gracilis is typically a plant that will deteriorate when it does not find enough nutrients in the substrate. It can use roots above the surface to overcome the problem, but once the plant deteriorates it doesn't do that any more.
I have experienced exactly the same problem in my tank, it is the only plant causing me problems (it did quite well the first two years http://home.scarlet.be/~tvelst/sugar22.JPG
). I added nutrients in the substrate and is taking a new start at the moment. ..