When I click my barbarian's Frenzy ability during combat, it doesn't always activate. Sometimes he just keeps on whaling away at his enemy, ignoring my commands. I sometimes have to click several times to get him to use the ability. It's not a delay, either--I've waited many seconds after clicking Frenzy to make sure of this.

I don't believe it is just barbarian frenzy that you are describing, if you play in slow motion with pause and track all your command clicks on paper I think you will find many commands you issue are lost dropped or ignored by scripted actions. Like whatever it is that makes everyone bum rush a target from time to time.