Check to see if it's holding an everstone. An everstone is an item that prevents the holder from evolving. Also, if you have actually encountered the evolving screen, make sure you aren't pressing the B button. Also, have you left it at the daycare? It will not evolve at the daycare, no matter what level it grew to. If that's the case, simply raise it up a level again and it will evolve.
It would also help to know how you obtained your Gible. Did you receive it in trade? If so, it might be hacked to evolve later or not at all.