If one toon gets feared or something alike than all other toons wont see his target untill he re targets something. So if you just assist some toon make sure to re target something on that toon after the fear.
I verified that this is not the problem, as I can retarget with my main, and smack away with my dps button, but still cannot get clones to assist/attack. I will try out the cleartarget thing this evening.