I keep trying to sneak attack this bandit chief but it just WON'T WORK ! I've done it nearly 10 times. I walk up behind him, in stealth (like I always have before), with daggers, and when I hit him I don't get the 15x damage mod (yes, I have the talent). I can't hope to kill him as he just 1-shots me every time he takes a swing at me. I know I'm in stealth, and I know the attack hits, as it just barely puts a dent in his health, but there's no modifier!!!

He might be detecting you as you're swinging. I have this happen a lot with higher-level Falmer. It's extremely frustrating. It goes from a slit to a full-open eye in a fraction of a second.

I think you may be right... well, time to get out the bow and kite forever since I have no hope of killing him in melee.

Leading them into traps works too, or look for hanging firepots to knock down on them

^this

Paralyze potions help alot against those mofos as well.

I enchanted a Blades sword with Paralyze. It works remarkably well. Especially good for Giant killing.