You must be connected to the internet and they only appear at 8-9 PM(relative to your timezone, btw). It's a silly system built to be annoying. Especially some stones are in the version of the game the pokemon is for isn't in (Aggron's stone is in Y, Tyranitar's stone is in X) forcing players to trade.
Oh, I was about to mention that, I actually had my friend look for Aggronite at the appropriate time, connected, and it wasn't there. It doesn't seem to work for everyone which is annoying. Just try again tomorrow, it may show up.