Hmmm, that's really odd! How much water are you giving them at a time? Are you removing all runoff water immediately so it doesn't get sucked back up? Maybe something to try is to give them a little less water at a time, in a circle around the plant. That way the whole medium isn't getting soaked. Another idea is to blow a fan over the medium itself, so it's drying out the top soil after you water. Once the plant starts growing fast again, you could go back to normal watering because it'll be drinking a lot more so the medium won't stay wet for long. The only reason I say anything is because cannabis tends to grow faster when it gets wet/dry cycles every few days. Honestly though they look really great!