The only mod I use is Enderal by itself, but well, while playing with the skill I noticed that Holding the "shout" button after marking enemies to apply the effect seems to cause this problem.
What I did to get it working most of time was Holding the shout button to apply the third tier of the spell & slowmo effect, then tapping the shout button after marking the enemies rather than holding it again.
It still "fails" sometimes, but it's nothing compared when I hold the shout button after marking the enemies. Despite of they attacking me sometimes (probably because of some companion/summoned creature already attacking them), the enemies still explodes instead of dying with the green gas effect attached on them.. Perhaps this is the reason I started to have this problem after I put a second mem point in the spell.
Anyway, I've opened a ticket in the bugtracker before creating this thread, but I was wondering if I was doing something wrong as I accidentally skipped the tutorial texts of the spell during a battle against some wild mages with those goddamned fireball staffs. IMO, it's the most badass memory spell of the game.