First make sure you are looking between 8PM and 9PM, it cant just be anytime of night. If you are searching during that hour and still not finding it check your DS clock as it may be wrong. Finally I dont know if you mean by "I have beaten the Elite 4 and everything", if you actually got your Mega Ring upgraded as thats the part you need to start finding the mega stones. Another note is that you may have already picked it up and forgotten about it, check your bag and if its still not there go to the location of the other stones and see if you can get those. If you can this probably means you picked it up and gave it to a Pokemon or overlooked it in your bag.