I suppose it's a glitch or something, because there is no reason it shouldn't evolve unless the previous trainer gave it an Everstone and then removed it beforehand, then it's normal for it not evolving, and in that case it won't ever evolve.
It evolves into Monferno starting at level 14, which evolves into Infernape starting at level 36.
As you know it should have already evolved.
I would suggest breeding for a Chimchar egg and then leveling it up, then it should work normally.
Hope I helped!