I hired an act2 prayer merc for my sorc and the aura either stopped or never worked (I haven’t notice). I started many builds of all classes and, as far as I know, it’s the first time it does that.
I hired another prayer merc to no success.
The merc (and sorc) have basic magic and rare item. Nothing that provide him another skill or aura or else.
Sorc is level 14 and merc 13.
My heal don’t change and I cannot see the aura around his feet.
He has do attack, than the aura gets activated.
Yeah I figured that out. I bet I just never noticed. It’s a bit weird. Why not make it active by default rather than activate it after a quick fight.
The issue is that activating the aura is considered an attack, so the Merc AI can’t do it until it’s in combat. The mercenaries just weren’t coded to support having auras on all the time like monsters with keyword abilities. They’re far more like players than monsters, so we get this limitation but in exchange they can be affected by equipment.
Side note: Bosses talking to you? Also an attack. They need line of sight and a certain range to activate it. 
Yeah TBH I don’t care of the mechanic. The problem I’ve seen recently is that it can take a while before the aura is activated. Even if the merc take some hits. Typically it goes fast but not always.
My merc is in combat and after fighting for about 15 minutes the aura never gets activated. Is this a bug?
Which aura is it?.
sdkjcndvckls
Keep in mind that the Merc’s chance to trigger the aura is also level-based, so the higher the level the faster it turns on. This is more noticeable when you’re getting rushed to A2 and can buy an A2 Merc with a level in the single digits or low teens. Having to wait several minutes for it to finally kick in at these levels is not unusual.
It’s the other way around, their chance to activate their aura decreases indirectly as they level because their chance of using Jab increases.
I haven’t been able to find the exact formula but I did find a site that says "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."
My own empirical experience says that low-level A2 Mercs are always slower to turn on their auras than higher-level ones, whether its a formula or a bug. A 2.5% chance at level 90 seems hard to credit as it never appears to trigger that slowly. If someone has the wherewithal to test that out I’d be curious as to the results.