OK, so Ive checked everywhere and everyone always says that you have to get it fighting a little bit to start it up, but honestly, the auras on my Act2 mercenaries are active only about 5% of the time, if that. When I am able to get Holy Freeze to work its such a massive boost, but it doesnt stay that way.
So you hire an A2 merc in NM with the holy freeze aura and from there on it should have it active, no? Im obviously missing something.
Maybe this will help (from Wiki):
Act 2 Merc Aura Activation
Act 2 Merc auras do not automatically activate when you hire them, or do they become active when you leave town. The Desert Warrior requires a little stimulus to use his aura and this means getting him involved in the action.
Basically, the Act II merc will only activate his aura when he has to pick an attack. The chance of activating his aura decreases every level up, to around 2.5% at lvl90+. So if your character kills everything before the merc gets a look in, the aura won’t activate.
It can be a little more complicated than what is stated above and there do seem to be instances where a hired mercs aura will simply not turn on, or at least take a very long time to do so. Simply returning to town and then letting your merc kill more monsters should normally solve such problems.
https://diablo2.diablowiki.net/Guide:Mercenary_Guide_v1.10
Im having this same issue. I have tried many of the tips suggested. I have ported back and forth from town. I stand still and let my Merc engage and kill 10+ monsters on his own. I sit there and wait. Still no Aura.
Did this just get even worse from the latest patch? I had definitely seen the aura working majority of the time prior to this.