You should be using pH'd water without any nutes. Dropping pH is usually the result of overfeeding. The plant sucks up the water and leaves the excess nutes, resulting in a pH drop.
If you have been feeding only pH'd water and no nutes, I have two recommendations. First and most important is to calibrate your pH meter. If it still reads low, flush a little more aggressively, but you'll have to walk the fine line between flushing and overwatering.