If you are doing it, I'm not sure how. I tried doing it in lowbie pvp the other day to see how on earth it may work, Perhaps it just bugs out sometimes.
I think it's a bug, during the weekend I had a sorc cap a door in voidstar without the bar appearing on my screen, but seems to have appeared on everyone else's since they screamed at me in chat. Would be interesting to see if no one can see the bar, or some can like in my one. In hindsight, my fault for wondering what he was doing rather than shooting him