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That is simply the way mobs are set up on our server. They despawn either after a certain amount of time or if you go too far away from their original spawn. To combat this, you either have to stay closer to "their area" or deal more damage. You can increase your damage by for example using a different talent setup, using different spells, or getting better gear!
I hope this helps out! If you have any further questions or need further help, feel free to reply below.