Happy Frog is a decent soil, right out of the bag, and also has plenty of nutes, so it's likely not the problem. Okay,...you say you water them twice a week. When you water them, do you water heavily, making sure every single crumb of soil is wet? That's critical. Think about it raining outside, and try to water like that, evenly, and completely, when you water. There is no risk of overwatering, in one session, so make sure you water it til the soil is saturated, and a little runoff comes out the drainage holes at the bottom. That soil has plenty of perlite for aeration, and as long as you give it a couple days to dry out afterwards,(til at least 50% of the soil is dry. I shoot for about 70-80%, which is a bit more ideal, but not critical) you won't have ANY overwatering problems. I suspect that you are underwatering, leaving areas of the soil continually dry, starving the plants. Rememeber,..dry roots aren't feeding, they're waiting for moisture, so they CAN feed.
MJ plants love water, but they'll also suffocate if you don't allow them to 'breathe', in between waterings. That's what overwatering is...a failure to let your plants breathe, and has nothing to do with how much water you pour in them in one watering. You could give that little thing a whole gallon of water, each time you water, and as long as you give it time to dry out afterwards, it'd be perfectly fine. Moral of the story...don't be afraid to water them heavily. I see it WAAAAAY too often here, that growers get scared of watering their plants, thoroughly, 'cause a couple noobs who messed up their own plants, scare them from doing do. Grr....LOL
Your soil is fine, and your PH is fine, so as long as there's no major temperature problems, or something else way off, I'd have to say you need to tweak you watering habits. Get that right, and you should have a nice harvest(with enough lighting.lol).
