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Could it be a range issue? I find I often have problems with Radial Sweep because the range is shorter than the animation seems to indicate, and is particularly unpredictable with differences in elevation.
EDIT: Having tried it out, I think the problem is elevation: slight differences can cause it to miss. It's a common problem with short-ranged AoEs, it seems.
But its a big AOE...If I recall its 5 meters and I don't think its elevation because I can stand perfectly still letting a mob beat on me and hit him once with sun shield then just flat out miss the next time.
5 meters is really short. Don't quote me on this, but I believe that is melee range. I know that I was able to shield bash mobs from the same distance my wife could lava whip them.
What I'm getting at is melee-range AoEs (which means 5 meters apparently) play very badly with even tiny differences in elevation. Even if you're right next to them, if they're slightly higher/lower than where you are the ability misses.