Make sure your gear is up to date. If it is, then either something you are doing is wrong or someone is doing something stupid in your party that is denying you aggro. Make sure you are going in with lots of flashes and circle of scorn, fallowed by the rage of halone combo. Then check in on any other mobs to make sure you aren't about to lose their aggro. Also I'd keep an eye on your healer to make sure they aren't over healing before you have a chance to establish threat, and ranged dps aren't attacking before you.