Sometimes I use new malts, and get a surprise. Then I kick myself say "should have done a minimash!"
Yup! I have been having a higher pH issue in my lighter beers with the base malt I have been using from a local malster. It is higher than what Martin's program predicts and I usually have to add some acid malt to the grain bill to bring the pH in the desired range for the beer, specifically my Amarillo IPA. I calibrate my pH meter once a month to remove that variable from the equation and usually get a couple years out of a probe before I have to replace it. I used to hit Martin's predictions right on the head when I was using a different base malt some years back so I suspect the difference is the local base malt. The local malster is going out of the malting business so I will be switching to another type of base malt, probably Rahr when I finish this bag. I will see if that makes a difference.
Additionally, I treat my mash liquor (R.O. water) with the necessary minerals before doughing in and only adjust the sparge liquor with Phosphoric Acid to bring the pH in the same range as the mash pH.