Gamers have found out that completing the Pokedex is a great deal harder in Pokemon X & Y as any Pokemon that is brought in from pre-gen in the Pokedex is not going to be counted.
One gamer told us about this and he became a victim to it. He got all the certificates along with the oval charm but he didn’t complete his Kalos Dex. This was due to the fact that Buizel, who was an older gen Pokemon from GTA. He posted a screenshot of it.
However it seems that the fix is easy. A player said that if you catch the Kalos version of the Pokemon, this will solve the issue. The Kalos Dex can only be completed if all Pokemon have been hatched or caught in the Kalos region.
So the Pokedex in Pokemon X & Y is very hard to complete. Previously the Pokedex could be done on your own now it is advisable to have friends working with you.