Yep PH is first, and foremost. You are looking for a finite answer for a problem that could be a plethora of stuff. I will say however that typically in a grow situation like this there is a tendency to use as little soil as possible, and the problem you can run into a lot is peat in the soil breaking down rapidly, and causing the PH to plummet rapidly in the middle of a grow. Especially when the soil is drying up between waterings.