Am I not cool enough to do critical hits?

Sometimes I can do a critical in VATS, other times the option is greyed out even though my bar is full. I've tried spamming spacebar in between choosing the body part and firing but without success. I'm pretty unlucky (4 to be exact) but I was under the impression that luck dictated how fast your bar filled rather than whether you could actually make a critical or not.

yeah its a bit tricky i think you have to activate it somehow, i just hit the A button on my xbox controller and it hilights it sometimes