Priest Raid Healing Question

I'm totally new to raid healing, and I just had a few questions for some of you veterans. I consider myself a decent player, and have no issues in 5-mans, but when I went into LFR for the first time I found myself running out of mana very quickly. I've since gotten several gear upgrades, and run a couple more LFR's and while I seem to be lasting longer, I'm still running OOM very quickly and usually am 5th or 6th on the healing charts. I guess I just want to know if I'm doing something wrong? At the moment I'm using Cascade, Circle of Healing and Holy Word: Sanctuary on CD, and I'm using Prayer of Healing as a filler. I find myself spamming PoH for the most part and run out of mana very quickly. I'm using Mindbender and Hymn of Hope on CD to try to keep my mana up as much as possible, but it just doesn't seem to be doing enough. Is mana an issue that other Holy Priests, or other healers in general, are running into or am I just missing something? Any help is appreciated.