Lissandra Ult. bug?

I pretty much only play Lissandra and over the past few weeks enemy champions have just been walking through her ultimate. In some instances the ult. animation plays out as the enemy seemingly runs through it untouched to kill me. People flashing out of it after frozen with no spell shield or but removal item. main offenders have been yasuo, yi and zed. Over a year of play and I have never seen someone go through her ult. and never seen the animation play out with nothing inside until now. maybe its an item bug since I cant recall this happening with against an AP champion?