To lower your PH, as TES says, you need to lower PH of the water you use.
If I understand correctly you may need to flush the plants first, I'm not to experienced with soil as I grow DWC, but you will need to flush the soil with 3 times as much PH balanced water as your bucket or pot will hold, for example if you have 10 gallon buckets you will need to flush them through with 30 gallons each, a 5ltr bucket then its 15 ltrs of PH balanced water, then start again with water and feed.
Do you use tap water or distilled / RO (reverse osmosis) water? if its plain tap water, let it stand for about 24 hours to give the chlorine a chance to evaporate, tap water comes with all kind of nasties in it, best off using distilled or RO water if possible.
I'm not sure if its the same for soil, but don't use the 'recomended' dose, I would say start off using about half to two thirds, and increase if needed, and measure the PH of the water AFTER you have added the nutes.
If I got any of that wrong, I am sure someone will correct me, and if it's smoking mom I would like a proper correction please, whips and chains for being a naughty boy!