Really? How about inadequate water, too much water (even worse), bad pH, using non-sterilized utensils to make the cutting, not using rooting powder/gel, stressing it out with too much light at the beginning, overfeeding it... I could go on - but why are you asking this now? It seems you're a fairly good grower by the looks of your diary ... are you wanting to give a certain desperate someone a point in his failing effort to take first prize in a contest or what? You did that with your OTHER stupid question about the best time to plant autoflowers 3 weeks ago...
A lot of us will be watching to see what answer you choose....