>When you have achieved the ability to Mega Evolve your Pokémon, you will be able to get the option on the touch screen of the Nintendo 3DS while in a stage. Once you reach Rank 33 and complete the challenge, you will receive the Stone Shop that lets you give a Pokémon a Mega Stone for 5 Poké Diamonds. Then, if you press the button with a Mega Evolution-capable Pokémon, a cutscene will occur where your Mii activates their Mega Ring and the Pokémon will Mega Evolve.
>When your Pokémon does Mega Evolve, it'll create a small shockwave damaging any Pokémon in the immediate viscinity, and will increase your Pokémon's stats to match its Mega Evolved form. This lasts for the duration of the match.
Source
Are you sure it mega evolved? If so, then did create that shock-wave? If yes, then it must have mega evolved, and its stats must have been raised. Did you check the stats before or after the battle, assuming that like most cases, you would wait for the battle to end? That may be why you didn't see the changes.
