Hi Sunfire
If its the same plant Aponogeton rigidfolius, this plant would be considered a highlight plant it would really need somewhere in the region of 3 WPG and regular fertiliser to thrive, i would say its dying off because of this, perhaps look up the above name and see if you can find a picture and see if its the same plant, if it is i would just remove it from the tank, you would probably have to upgrade your lighting if you want to grow high light plants.