I don't personally play tanks (except in small friends-only groups where we're low-manning older content), but I suspect that your boss rotation is where the issues typically come up.

Provoke is the ability I believe you're using, to place yourself on top of the enmity list. I may be mistaken, but I don't think this gives you a fixed amount of enmity - it just makes sure you've got more than everyone else. So if you're opening a fight with it, that's not very much, and healers or damage dealers can easily rip it back away. I'd recommend on bosses or single-targets that you open with Shield Lob instead, which gives a significant starting amount of enmity right from the beginning (and use Provoke if you ever lose hate for some reason).

Also, make sure you're using Circle of Scorn, too. Between that and Flash you should be fine, though Shield Lob again still has a use here, if you happen to miss one of the critters (the range on both Flash and CoS isn't all that big).

- Suggestions from a lower-level, very casual tank.