As far as I can tell, there must be something wrong with your plant. It can either be light, the absence of fertilizers, or both of these factors, or it could be something else entirely. You can take some remedial measures in this regard if you wish to do so. As a first step, you should remove the yellowish or brown leaves from the plant because they have already started to deteriorate, and rebuilding them will require a lot of nutrients and energy. There is no doubt that a root tab is one of the most important things to have in the tank, especially if you have that type of Amazon Sword or other aquatic plants in the tank. Another way to revive yellowish and brown leaves is by injecting them with carbon dioxide, which is another solution to revive them. If your fish are exposed to too much carbon dioxide, they might die as a result. I would appreciate it if you could do so accordingly, and I hope that would work for you.