I already know that you cannot cast this skill onto enemy which is immune to stunned.
(Unlike other similar skill, you can cast it any time just if enemy is immune to stunned they will not be stunned)
But I find sometimes you cannot cast this skill even the enemy is not immune to stunned.
For example, I tried this skill onto a troll, it successfully stunned the troll at the first time, but then I found I cannot cast Rune Cage onto it for the second time, even after 1 minute.
But if I ran away and let the troll reset, I can use this skill onto it again, but only for the first time hit. I cannot cast it for the second time again.
Is there a bug or some mechanic I don't know?