Without knowing what you're pressing when, it's hard to say where you might be going wrong.

askmrrobot is a great place, with an easy to use interface that will show you how to maximize your enchants/reforging/gems...and show you what pieces you could and should get for upgrades. It also has a nice "this is an optimal talent tree" tab. Although, for arcane, we have a lot of extra points to wiggle around within our own tree.

don't forget to make sure you're buffed with food/flasks.

and, as always, the problem could be your rotation. i'm at 357ilvl, doing between 17-23k sustained dps, peaking at about 49k. some tips i would suggest:

make a macro of ALL your cooldowns (except timewarp. people get mad when you blow it at the wrong time).
watch your cooldowns. you can usually squeeze out an extra cooldown on heroic fights, and can definitely get a couple of them out on raids multiple times.
watch your mana. the more mana you have, the better...but don't try to keep it topped off at all times. from what i've noticed, 60-80% mana is a nice safe spot to get lots of dps without sacrificing arcane blast casts to barrage and save mana.

my damage usually turns out to be about 88-90% arcane blast, 3-5% missiles, and 1-3% barrage and flame orb. i rarely use barrage...unless i want a killing blow or am on the move.