Help identifying a pH drop

[tait]

hello! im having an issue that my ph is continually dropping, so far i poured out half of my fresh batch of nutes thinking it was my ppm, which was at 1860
and refilled it with normal water at 400ppm
maybe about 3/10 gallons.
yet my ph is still dropping?
what you see in this photo is the automated system I am using, Mycodo, the green line expresses my pH and the blue lines are 1ml doses of ph Up being dispensed, continually keeping the res above 5.6 (and below 6.2 with a pH down pump)
i know this isnt healthy for my plants to sit in this stasis, can someone help me diagnose?

 

[Cashmeh]

Picture of roots and plant, what ppms did you start at, what week did you change them at, what week are you currently at in your grow. Do u use ro water or bacteria? What type of system do you have them in. What is your brand of ph up and down.