You need to pH your soil itself doing slurry tests then adjust your feed accordingly, the same thing applies in soilless systems that don't have frequent fertigation with runoff. Rootzone will be telling another story than your input water. My guess is you were accumulating citric acid in the rootzone and created an overly acidic soil/rootzone.
I've had this happen in coco where I wasn't doing runoff, I would have to regularly check my rootzone pH & EC (PPM) to make sure things weren't toxic, now that I have heavy runoff in soilless (coco) my EC & pH stays pretty balanced through the whole grow. To adjust an acidic pH of for example 5.0 I would have to feed a couple times with a 7.0 solution to hit 6.0 at the rootzone. You could do a slower adjustment but I haven't noticed a benefit waiting to solve the problem.
Mix 1:1 soil:water and stir, let sit, have a smoke, check the pH of the dirty water. That's what your roots are actually living in, not what you pour into the pot, also, Reverse osmosis water or distilled water will always be neutral or 7.0 pH, this is because there is nothing in it, assuming your filtration system is working properly and your meter is calibrated.
