Question Regarding Death Attack

[Grunk]

I have a Monk/ Warlock / Assassin.

I have Hideous blow up and I use my fists. I have tried attacking with and without Flurry of Blows, I sneak up on an enemy and my death attack does not fire off at all. There is no saving throw when I hit or nothing. No extra damage. I know i have another character that is Rogue/Warlock and the Sneak attack works fine. What could be the problem?

[Luna]

Anyone know about this case?