well I am looking at spell file and if I am reading it right, Atone only has 1% chance to wipe hatelist.
Cause Blanket of Forgetfulness (chanter spell) has 2 wipe entries of 20 each, which means its 20% chance each, which leads to 1% for Atone.
at first I thought thats BS cuase what kind of good does 1% chance do to a 60 mana spell, so I opened up Allakhazam and umm, indeed Atone only has 1% chance to wipe hate, thats why you have to cast it 100+ times
Chanter's spells usualy wipe agro on each 3rd -5th try cuase they start from at least 20%