What’s their knownlist size set to? I can only assume that it’s 1000-1200. It should be somewhere around 1500-1750 (considering some sit behind you). The problem is that many times, they’ll simply ignore a target being attacked by you (with staff) at the edge of the 1200 range.
If it’s not a knownlist issue, then it’s just poor AI. But I hope it’s just a simple fix like this. Should be applied to all other minions as well (pets, etc).