Have you upgraded your Mega Ring yet?
After you finish the game, head to Kiloude City and battle your rival. Once you do, talk to Professor Sycamore and he will upgrade your Mega Ring so that you can detect hidden Mega Stones from 8 PM to 9 PM so you can go hunting for all those Mega Stones!
Also, the game can tell when you adjust the time from your 3ds settings, so you need to actually wait until 8 PM for them to actually appear.
Hope this helps!