pH will not stabilize

I have a Bubbleponic system that I purchased several weeks ago. Ever since I started the system, I cannot get the pH to stabilize. My tap comes out at around 7.2. I add the down and check back a few hours later and get it down to 6.3, but by the next day it is back to 7.2.

Any ideas on what the problem is? Could it be sub-standard pH down? I am using the pH up/down that came with the kit (dry).

whats the ppm of your tap water? I solved my ph drift with a RO filter..

I do have an RO filter but my PPM is the same straight from the tap at 104.