I recently took off my global critical rating gear to put on my block gear that only has critical rating on myth and nothing else. Ever since that when I use vengeance (20% critical aura) I never ever had critical a school outside of myth. So do I need to have at least 1 rating on a school in order to perform a critical? The vengeance is only a 20% boost so there's no critical rating increase.
It's just bad luck. 20% isn't that much and therefore highly unreliable. I have 55% critical on my wizard (while maintaining 38% block) and I can miss a critical 3-4 times in a row. It's theoretically possible to never get a critical as long as you don't have 100% critical.