It could be a number of things, as well as a combination of a few: overwatering, underwatering, ph imbalance, cool environment, etc.
If it at all helps, my initial reaction was that of overwatering.
However, to get an accurately assessed judgement, you really need to supply some more information here. Strain, lighting, watering methods, water used(amount), type of water(sat out/dechlorinated/spring/distilledph level), temperatures, humidity, length of growing, clone, seedling, growing environment, etc.
If you supply that information I think you can get a better response.