Igglybuff evolves off of happiness, so go to the friendship checker in Laverre City and check its level. If its not max happiness try the many methods to raise its happiness to max. Level it up after that and if it doesnt evolve then something is wrong with your game.
Edit: Sorry missed the part of the question where you said you already went to the friendship checker. Anyways, it might be something to do with the igglybuff or the game itself. Try to get another igglybuff and make it evolve. If you run into the same problem your game might be glitched.