How are you doing your flushes? You need to give your plants 3x the amount of water as your pot size so if you're growing, say, in a 5 gallon pot, you need to "flush" with 15 gallons of pH'd water... if you use PH Down, get each gallon of the water between 6.0 and 6.5 and this WILL work. If you want to do things organically, there are recipes out there for using lemons or apple cider vinegar but I don't know those offhand because I don't, sadly, work organically. Anyway, mid-way through your flush, check your runoff... you should see the number going down - and then check it again at the end.
Always pH your water/feed! A pH imbalance leads to most problems growing weed...
Good luck!