Melee minions run around and can't find the enemy

When I'm in battle with barbarians, I have noticed (because I can see it then as there are only a few minions on-screen) that sometimes, a melee minion (e.g. a hammer user or an axe user) will not go straight to the target, will not receive any ! mark above their head, and will instead run back and forth with their weapon above their head. They sort of get close to the enemy/enemies, but never actually get close enough to attack. Thus, the unit is useless for combat! :/