Hello, when i PvE with my shamans, they always gets oom

It's like, I have water-shield and mana totems.
But when i PvE i only get like 230 Mana when im in combat, i've only stacked spell-power for maxium DPS and like Crit/Haste.

But, how should i make my mana now to oom all the time? i have 20k mana unbuffed and so.

And i saw that Shadow-priest does quite good in Raid, and they never gets oom because of Spirit+spellpower etc.