Spell rotation or spec might be an issue, as well as gear. It's difficult to pinpoint without more details.

My experience with mages though is that you're very crit-rating dependant for your DPS, and they have a much higher wind-up/tenderizing time than shamans. I mean, shamans just start dumping DPS to the point of being ridiculous. A mage has to start off with scorch. Then some 3 seconds FF bolts, then weaving your Pyros. I've you're fighting anything that takes more than 30 seconds to kill, a mage should perform noticably fine. Less than that, and you see cut offs and the like.

No idea what's wasting your mana though.