{sorcerer-Fireball-impact} do not correctly work

lately (2 days) i noticed that not all mobs strucked by a firball charged [and upgrade with skill impact] are knoked back and stunned (last thing more more important)

as skill says:
Fireball
Fireball: 116 Mana, 4 sec cooldown, 14m range, 84-105 damage, increased damage on a critical.
Engulf: Increase spherical area of a fireball explosion by 3 meters.
Ignite: Targets take burning damage every second for 5 seconds.
Impact: Enemies struck by a charged fireball will be knocked down and briefly stunned.

what's happend? why this bug is come out?
before all mob touched by explosion was litterally knocked down and briefly stunned.....now if i stun 2-3 is just a luck !

http://www.spacetimestudios.com/show...-miss-too-much

people reported a "quite similar" thing happened at the same period: since last free respec weekend of that time

but here the problem isn't missing target, all are hit but only few knocked down and briefly stunned

Some mobs are immune to stuns, some are immune to freeze and some are immune to root. Thats why you need all these 3 main aoe attack skills(fire, ice, clock) in your build.