KS on boss monster

[jeffvamp00]

Hello. I'm just wondering why I don't get the kill eventhough i'm the 1st one to get the boss monster.
The setup is, I go to boss location to farm blue chips. Only 1/8 on life left on the boss monster then a player shows up then hit it.
Then all of a sudden I can't do any damage on it and it says I cant hit monster a player is already attacking or some sort eventhough im the one to get it.

Please help. This happens most of the time and I don't know why.
See sample screenshot of someone stealing my boss kill by doing ks.

[jeffvamp00]

another 1

[jeffvamp00]

I now know how they do it. The trick is... If there are mobs within the range of the boss and the player use an AOE skill. That's how they steal the boss. The original player who hit it 1st will lose the authority.