i think it is extreme overwatering combined with other problems ascociated with the mg soil. if the plant/rootball is not large enough then it is gonna take a very long time for 5gal of soil to dry thoroughly. so if you are watering every 2-3days then the lower part of the container is always gonna be soaked which is an extremely bad thing for mj. when i repot into the 5gal buckets i put a 2-3inch layer of staight perlite at the bottom of the bucket to keep the plants from having "wet feet" . if you are unsure of how often to water you could always get a moisture meter, they're pretty cheap and well worth it for the new grower to help get an idea of what's going on in the soil. i used one religously for the 1st year or so that i grew.