Vampire Drain Ability

Athas24

Anyone noticing that sometimes your vampiric drain ability doesn't always work right when you try to use it? Sometimes I find myself clicking it 2-3 times before it responds on my Nightblade. It's only this character that I notice it on and its the only ability that I see the issue with. I also think that it's sort of dumb that the ability simply doesn't work on many bosses that are large etc. I understand that it may not stun them but not being able to drain them is sort of goofy. Also, if they stick with this maybe they should shade the ability when targeting someone who can't be affected by it or do something other than say, "This creature is too strong to be affected by this ability." In large scale battles sometimes with the response issue and the ineffective issue on certain creatures I often end up mashing my #2 button until it works or I realize it's not usable on the monster. Thanks for listening!