I was using distilled water with my nutes, and the nutes had no Mg or Ca. So even without the acidic soil that would lock these out I was depriving my plants and didn't realize it until it was too late. I found some magnesium sulfate at the local hydro store and thought that would help, but I was adding it to my normal soil watering and the pH was way too low for the Mg to get into solution for the roots to absorb. Foliar spraying with a more dilute solution of this did eventually help though.

If you can find a good digital pH meter it's worth the extra money. I got one of the cheapo Rapitest gadgets and it was a complete piece of crap (although a similar moisture meter from them seems to work OK).