There are a couple of common problems.
Number one is almost always the case, lack of light. Different plants require different light levels, if you have a plant that requires bright light in a tank which is not brightly lit, it will fade away and die. By bright, I mean bright enough. 2 Watts per gallon of full spectrum, (5000K - 6700K), fluorescent light turned on for 10-12 hours a day will give "Medium" light, assuming a regular sized rectangular tank. Deeper tanks and small tanks require more, larger tanks less. No amount of fertilisers or CO2 injection or whatever will help if your light isn't up to scratch.
Another common problem is plants sold for use in tanks are not aquatic plants at all. They drown and rot. Common problem.
Stickies will answer a lot of question.