I've had a lot of probs with my plants dying and I've found that they are affected by too much heat. My tanks are sitting at 28Celcius. and my plants die. I've noticed and asked at petstores where their temps are really quite low and their plants do alot better!'
My plants used to do a lot better in my coldwater goldfish tank than they've ever done in my tropical. at my lfs it says by each different sort of plant the min and max temps, water conditions etc etc, same as the fish, that the plants require and are healthiest in. You could look at this info on the net i imagine if your lfs doesn't do this.
Hth. sushi.