I don't know if this is your issue, but I had a similar problem with thunderstorm on my shamans. I spam my dps button, and then hit the round robin key for thunderstorm every now and again. The trouble is that if the selected user is on a global cooldown, then I don't cast thunderstorm yet. (I already had /stopcasting in the macro, so its not because of casting) I'm still not sure how I'm going to handle this, but GCD is evil, and sometimes causes problems.