Any body care to venture a guess as to what's happening?

I just switched from three Aerogardens to two six hole bubble tub DWC's and so far so good. Granted this is my first experience with this style of growing and so far I love it but some thing strange is going on in my reservoir.

I've never had healthier, happier plants but their nutes are suddenly turning acid. I'm using, exclusively, Technaflora's "Recipe for Success," (Which I've used for for over two years and been very happy with). Instead of my pH climbing as it always has in the past it's dropping radically. In the past I would set it at 5.8 and the next day it would be in the low to mid 6's. Now I set it at 5.8 and it's between the low 5's and the high 4's. I figured it might stop when I changed the nutes yeaterday but it didn't. The hydro store is stumped and I'll try to get an opinion from Technaflora and my hydro teacher when I see her, but have any of you had this experience? I can't really call it a problem since my plants a happy.

The only thing I can say is when this happened to me, it was the freaking ph pen. That pissed me off. Went through a bottle of UP before I figured it out. Did I mention it pissed me off?

for me everytime I drop ph its that my ph is stable but there is not enough food in the res. I add 1 gallon fresh water with 1 gallon of food and the ph is stable for another week. I also use the same nutes you use.