Make sure your tank is cycled (which I believed it is). In growing plants, you have to consider CO2 and lighting besides fertilizer. CO2 is essential for plants to grow (does not matter if they are slow or fast growing plants). Your sources of CO2 (your fishes and the atmosphere) may not be enough. But as long as neither of your plants is melting, they are good.
In addition, consider your lighting. Never use watts as rule of thumb for measurement. Watts is energy per time - its the amount of energy usage per amount of time. Many light bulbs/lamps nowadays are energy saver. So, they will have low watts, but will have proper lumen for growing aquarium plants.