[ME2] Flashbang grenade won't work in Arrival enemies consistently

I''ve just played Arrival last night and Flashbang grenade simply didn't work consistently against Project enemies. I've tosse grenades that blew right besides one enemy that was firing against me and the grenade didn't do anything to him. I was playing with Improved Flashbang with maximum radius.

Have you guys experienced the same? It's a thing about those particular enemies, or the inconsistency is consistent (a-ha!) with all enemies? 

thanks for your attention.