We can't really tell much because you haven't linked your armoury. However, I can say with some certainty that your doing some stuff wrong. I'll just list what I would change here:
- Mortal Strike?!!! UA build is not as good as it once was, you should stick with 15/5/51 since the devastate buff.
- Expertise is rather low need at least 6% to hit the dodge cap? (just using figures off the top of my head forgive me if I'm wrong)
- Start the fights with SS + Conc + devastate to 5. Establish early aggro first before applying your additional buffs/debuffs.
- Refreshing Taunts? You should only be taunting if you lose aggro or fight mechanics call for it.
- Using vigilence on the right person?
I'm sure there's more, that other people will pick up. But post up your armoury all the same, it will help dissect the problem.