Whats the pH of your water(BEFORE YOU ADJUST pH)? What soil are you using?
Nute lock out - you have mag deficiency - its because of your pH im assuming.
What have you been using to get your pH to 6.5? If you pH to 7.0 what is your run off? if your feeding 6.5 and getting 4.0 you have a difference of 2.5 so your soils actual pH is around 1.5 - I dont think anything except aliens can grow in this pH.
Your soil is probably if not definitely the culprit - if you can get some biobizz or plagron light mix and carefully transplant the root ball out of the soil its in spray the roots with pH 6.5 water and spray around the soil where your planting and get that baby in some stable soil - its not too late but the longer shes in that soil the more she will die
Also if you are using tap water what is the eC/ppM?
You could also try flushing with a flush formula like plant magic flush, fox farm sledge hammer, advanced nutrients final phaze - This could potentially take the cations out of your soil if they have been imbedded by synthetic chemicals or your tap water for example do this at a pH of 8.0 once then read pH run off if it rises higher than 4.0 then keep doing it at 7.0 to try get the run off more in the region of 6.3-6.5 if you can. If you eventually get to 7.0 you have leached all of the anions out of your soil and you can start a low strength feeding at 6.5 to put nourishment in the medium again.