FSM;3154854; said:
How much light do you have and what kind of plants are they?
This is one of the best questions ever to ask.
Here's a few good places for plant info and lighting needs.
http://www.aquaticplantcentral.com/forumapc/plantfinder/
http://www.plantedtank.net/forums/myplants/
http://www.plantgeek.net/plantguide.php
Wyldfya's sticky is pretty good too, but it lacks pictures, so if you don't know the name of a plant, it is not as useful. It can help you decide which ones you might want in the future.
Bear in mind that generally WPG is used to determine lighting strength, 4 is 'high' 2 is 'medium' and 1 is 'low.' The bulb should say on it how many watts. If it doesn't, search the brand name and model number and you will find it. Adding in the approximate size will help, bulbs are usually in multiples of 12'' long (length is based off of the fixture usually) If you don't know the name of the plant, search by type.
Generally, if you just have the lighting that came with the tank, it is insufficient... for all but LOW light plants. I had brown algae all over the walls of my tank, green spot algae on the glass and rocks... but the light was 21 watt over 65 gallons.
If you have repeated failures, get HORNWORT. I cannot stress this enough, hornwort is amazing and awesome! If you are impatient as to seeing changes in the plant, you will love hornwort. In low light it gets bigger every day. Within a few days it will be
covered in side shoots.
Don't believe? I have hornwort in 1.3 wpg and it is doing amazing. It isn't eaten by herbivores, but they might damage it initially, (it is brittle) especially new side shoots, which are softer. It doesn't mind being uprooted, as it never grows roots and can grow floated. It can also be weighed down.
HOW TO TELL IT IS HORNWORT
It has undivided leaves, in pairs like pine needles, that are in a whorl... and an abundance of sideshoots. It never forms roots along the stem like other stem plants. It is easy to mistake with other stem plants, though. It does not collapse out of the water. The leaves are firm to the touch.
Image borrowed from AquaticPlantCentral.com
My 2 cents.