Small bevil gear not turning

[RobertC]

Hi I just put a new mag on and noticed that my small bevil gear won't turn on it's own but when I use a socket to turn the mag it turns the bevil gear but seems to be a spot that has a little resistance when I'm spinning the mag and the small bevil gear does anyone know what it could be that's keeping it from spinning freely by just rolling the bike around I have the head off so compression isnt the problem

 

[Frogslayer]

There is a woodruff key under it that must have sheared. Or maybe never installed?