Your shield must be down? Why is that? I don't doubt it, but the second you attack (with any weapon other than spear, rapier, etc.) your shield is technically "down", isn't it? I haven't really noticed this nuance yet. Might explain why I suck at back stabs.
The backstab won't work if you don't put down your shield before you push the attack button.
Learning how to switch effectively between shield up and down is a key feature of the gameplay even if you don't backstab. It's useful to optimize your stamina, parry effectively and so on.