I'm pretty certain it will always at least stagger an enemy.
It doesnt sometimes, they receive damage but continue walking toward the backstab.
It's lag. Try to turn around real quick and then cast it if someone is chasing you. They will probably have their shield down since they will be going for the bs.
I do that, but they are sometimes immune to stagger altogether even though they take damage. I guess i'll just have to deal with the lag and try to cast wog a bit further away to give myself time to roll away.