My Merc don't actvate aura

It often happens that my Act 2 Mer activates its Aura only after a few minutes. Does this happen to you too?

Yes. They constantly switch between nornal attacks,

Jab attacks and aura activation (once activated, aura stays active). Probabilities are: 30/110 = ~27.3% chance for normal attack, 70/110 = ~63.6% chance for

Jab, and 10/110 = ~9.1% chance of activating their aura (I found this information in the Internet somewhere, maybe it's not up-to-date).

It does, yeah. Very annoying if your build depend on his aura.

Maybe @Nekeyby's info above is right, as I noticed my merc takes his time to active the aura until he starts getting hit at melee range — which means he's fighting back, so those percentages might be accurate after all.

The merc accompanying my summoning druid can take several minutes to activate his aura, probably because the pets are doing all the job for him.

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TLDR:
Merc needs to participate in active combat to activate aura. Not just witness, but actively attacking.
This is specifically for A2 Merc with native aura. If Merc has an aura from an item, that will always be active.

happens a lot if you kill too fast lol, the merce thinks "meh, this sorc kills everthing, no need to activate aura"
you can let him attack alone and will activate the aura in moments when he takes a lot of damage.