I think there must be something wrong with your plant. It can either be light, the lack of fertilizers, both, or even something else. You can take some remedial measures in this regard. In the first place, remove the yellowish or brown leaves because they are already dying, and rebuilding them will take a lot of nutrients and energy. In my opinion, there isn't much hope. A root tab in the tank is one of the essential things, especially if you have That type of Amazon Swords or other aquatic plants. Carbon dioxide injection is another solution that can revive yellowish and brown leaves. Your fish can die from a bit of extra carbon dioxide. Please do so accordingly and I hope that works.