Vi's "E" sometimes does not work

Sometimes when vi uses her "E" spell (I think it has to be the last charge or have her "W" attack speed buff on) the spell will give the sound cue of the spell activating but the attack will only count as a normal auto attack not hitting any enemies behind the target and not showing the waves of her "E". I have only ever been able to do this bug on The Big Raptor, The Big Krug, The Big Wolf, and Gromp. In addition, I have only ever been able to do this with the "sated devourer".