Yesterday I went to Valley of saints to test Banish Seraph, and from 50 times that I used the skill (I know cuz I've spent 250 soul ores) it only landed twice on yellow mobs. Check it pls.
Today i went with a sk friend in cata and with white mobs it was landing like 1/3 - 1/4, so the skill is working fine, try with mobs of ur same level.