Essentially you need more watts!
Now, this will depend on what tank you have. Does the manufacturer offer T5 lighting systems for the tank?
If not, it looks like you will have to DIY it. T5 tubes give a lot more watts for the same size (actually, T5 are thinner!).
How many tubes do you have now? If it's only one, is there a possibility of setting up another?
Also don't forget that plants don't just need light. They need ferts and Co2!
Flourish Excel is very good and is liquid form, so you just dose the tank every day or two. Red plants need Iron, so Flourish Iron can help there. You may also want to use Fert tabs under the plants. You can also get what looks like gravel, but it's a Fert. You use a base layer under your gravel.
Co2. You can get Co2 Injection systems to automatically insert Co2 into the aquarium. This is the best method, but is pretty expensive. It's the most controlled method though. You can buy a Nutrafin System, which used warm water, yeast and sugar to make Co2 and using a bubble ladder, you can apply it to the aquarium. You can also make a home made version, but if you do be carefull with the amount of yeast you use or it will end up in the tank. Don't forget though...... Co2 alters you PH of the water, so too much could be a disaster.
and that's it.......in brief! I suggest you have a look around on the net and do some reading. A sucessfull planted tank is a lot of work.