Title says it all, please report posts like the parent.
Do you have a castbar that shows you the ticks of your Drain Soul? When channeling Drain Soul and you have to cast something else like Haunt or Bane of Doom, you should always cancel Drain Soul exactly after it ticks the next time, as to not waste casting time.
Maybe your fire mage and arms warrior time their big cooldowns to fall in line with the execute phase? Affliction has a strong execute phase, but no really strong cooldowns.