PH Problems

Hi,
Im using a DWC system to grow a few tomato plants and Im having some trouble with the PH. Ive added alot of PH down and it stays around the same everytime i re-check it (around 6.5+). Is there any reason why it would do this. Im using fox farm nutes (grow big and big bloom at about 1/2 strength) and some GH Floranectar at a little less than the recommended dose. Im also using GH PH testers with the liquid...Any ideas?

All of your nutes typically raise the pH.

how big is your rez? and nutes decrease your ph.

I have a 6 gallon res. These nutes seem to raise the PH and this is the first time ive had this problem. Im thinking it might have to do with the floranectar because before I started using that the PH would adjust very easily and was sensitive to just a drop of + or - ....