Dont Worry, be happy. The Ph wont stay still if youre using any kind of "recycycling" system simply due to the salt deposits formed and left behind by the constant wet/dry process. Advanced Nutes are said to buffer themself.
Thats the simplest way to putit. You flush to wash away built up salts from your nutrient solution. The salts build up in the rootzone and in the media. So, if you add nutrients to this flushed out water, you will be putting it right back if you reuse the water, PLUS youll be adding MORE salt...