My Monferno is at level 48 and is still not evolving. Why?
There may be multiple reasons why your Monferno isn't evolving...
It is holding an Everstone, which makes the holder not able to evolve until it is taken off.
You are misreading your Monferno's level and it it actually level 18 or 28. (Probably not the reason)
Your Monferno started to evolve but you pressed the B button and it stopped evolving, and you repeated this until it was level 48. (Also unlikely)
You have an Infernape on level 48 named Monferno. (Also unlikely, but possible)
Your game is gliched
Hope I helped!