defo a ph problem im going through the same thing.
i was adding stood tap water at 6.4 ph but the nutes in the soil were knocking it down meaning my run off was in the 5s!
ive added some garden lime to the top of the soil(raked it in),then watered once with lime water until my ph up arrives then ill be watering with ph'd tap water of around 7.8 hopeing my run off will be around 6.5......at the start of my grow the least i knew about was ph and its the most important thing to study in IMO.

have you noticed slow growth?
i have plus there grow wider rather than taller.