I was running Koxx Browns with a smooth Koxx rim and plenty of tar, my brake was incredible (in my opinion), it locked really well and held every time. But after the DJ ride (when it rained) I swore I was going to get a grind, as I was sick of my brake being useless at the thought of water.
Anyways, I just took out a grinder (with tar still on the rim) and dug the b*****d in basically, nice harsh grind was the result. After a few minutes of riding around, bedding in both my pads and the grind, my brake was insanely good.
Maybe it's your set-up? My pads are set about 2-3mm from the rim and are perfectly square (bled with water, if that makes any difference). Safe to say I won't be going back to tar now ;)