Without a diary and not knowing exactly what you may or may not be doing, this question is almost impossible to answer precisely.
It could be many things, soil pH being very wrong as one obvious suggestion, but there are also many other factors involved..........poor drainage, cold temps, incorrect lighting, over watering etc, etc.........but without a diary...........
Unfortunately, being 6 weeks and autos, these plants are not worth continuing with and starting over may be the best option.
Try reading the "grow guides" at the Mandala Seeds website before starting again, these guides will give you some great ideas as to what to do and what not to do.