While you did not provide a lot of details i can tell you Warriors with defiance have one of the easier times keeping AoE aggro.
Always look at your threat meter... this is a must for all classes but a tank if your red is blinking you are in danger of losing aggro.

Assuming you dps isn't all focusing random mobs you rarely have an issue with aggro until max level when ilevels could make you struggle with aggro. When i played Warrior I would overpower twice before i start rotating into hate combos. Cycling through the mobs on you two enmity attack helps quite a bit. If i know we are doing massive aoe having flash as a cross class can be helpful for helping to keep hate while not expending TP. Past this with what you have said it is too difficult to know any other issues you may be having.