The only thing that would prevent either of them from mega evolving is that they're not holding their mega stones.
Either that or maybe you haven't received your key stone yet from the gym leader Korrina, which allows your Pokemon to mega evolve in the first place.
In your case, you would need your Lucario/Charizard to hold the Lucarionite and Charizardite X/Y for them to mega evolve.
If you don't have either mega stones yet, this might help.
Hope I helped :D
