Backstab frequently misses - why?
You move slower in Stealth, and slow due to combat, while they may have Swiftness. Try using Signet of Shadows, it helps a bit but still the movement difference is pretty noticeable and you have to adjust. It basically means using a Dancing Dagger before going in for the C&D / Backstab.
That, or you could be stepping through your enemy, or you have Melee Assist still turned on in the options – turn it off, and be in the sweet spot ‘inside’ the enemy’s collision range.
Or this could just be lag.