Most likely if you are in the US, the bulb is only a 15 watt bulb. That seems to be the standard here. If you have changed the bulb, I have no idea what it could be but doubt its over 20 watts due to buying bulbs myself and finding nothing under that amount for that size tank lid.
In a 10 gallon tank the bulb seems much brighter than in a larger tank, but its only because of its size. We have successfully started plants under this wattage bulb, but have then transfered the plants to much larger tanks with greater lighting.