Plants will not use nitrates if they have ammonia available. Plants and bacteria are competing for the little ammonia produced daily by the fish. A plant, just because is there, will not consume ammonia (or nitrate). If it is not growing, it means that it will not need nitrogen to make plant mass. Nitrogen (in the form of ammonia, nitrite or nitrate) is no longer in the water column, it's in the plant. Wait for the plant to rot and the nitrogen is back in the water column.