Soil PH wont be the problem mate, how do you know its deffo not nutes?? some of those nute release soils can overfeed your plants or a "hot" spot in the soil can cause a upfeed of nutes thats not required and burn your plants .... its really really hard to cause lockout in a soil grow because the soil buffers the PH , id never use a nute enriched soil for this very reason ... best advice i can give you is run 2 x the amount of water to pot size thru the pot ... ie, 6 ltr pot run 12ltr water thru it. after that let it 90% dry out (3 days or so) then start to feed your plants with a half amount nute mix but only enough so the pot dries out in a day or 2 and then after 3-4 days increase the nutes again , you should see improvement in a week to 10 days