Have you overwatered them??? It's a pretty common mistake. What are you growing in? Indoors or outdoors? Try to keep in mind, the the simplest solution that answers all the questions at hand is usually the right solution. If they are indoor or outdoors and the humidity is through the roof, make sure that you are not using more water than the roots can take. The high humidity will slow down the plants ability to move water and the soil's ability to evaporate water. Good luck!