Why does mez sometimes seem to never blur a particular mob?

[unsunghero]

I never bother to spellswap mem blur (I don't think I even bought it), because mez is normally so effective at blurring. I hear that your charisma helps, and I have about 230 buffed so mez blurs way more than 50% of the time it feels like

But occasionally some mob will seem bugged to where it is just immune to blur from mez. I think I've attempted like 15 times in a row before just giving up and dealing with the mob in some other way, and this has happened on a few occasions

I am not standing in aggro distance, nor is anything socially aggro'ing it. I also haven't put any debuffs on it, although I have countless times been able to blur mobs after hitting them with wandering mind. It's a great way to get some mana on the go, just mez a nearby caster, then wandering mind it, then mez it again. You should get back more mana than you spent on the mez's. So I don't think having a debuff up should prevent mez from blurring, only a damaging dot should do that, but regardless, no debuffs up sometimes and mez still won't blur

Is the game RNG seed just getting stuck? Or am I just paying more attention to when I get really incredibly unlucky?