Yep, it annoyingly seems to not trigger sometimes. Interestingly, I'm doing a run now without using Illithid powers at all and it seems to be triggering more consistently. I always suspected Psionic backlash (or too many reactions on a unit in general) caused a conflict but could never prove it. It may be some other strange coding witchcraft unrelated to Illithid powers though.
I think it might depend on the unit's distance and the game misjudging that distance and thus miscalculating whether a spell is in range of being counterspelled. Maybe due to uneven terrain.
There's a fight in act 3
where you confront an assassin who is scripted to use a special dimension door ability to dip out and escape the fight. I always prevent this by huddling multiple counterspell users around him as close as I can and the counterspell reaction has never failed to trigger.
It's those long range counterspells (though still well within range of what should trigger the reaction) that sometimes do not trigger, in my experience.
