Talk:Lissandra/@comment-26170076-20150310120736/@comment-24613667-20150316022434

"There is a ~0.4 seconds delay before the main target ( if it's an enemy) is stunned and damaged. If a champion becomes untargetable in that delay it will be able to avoid the damage and stun. The ice field will be formed anyways."

Basically, you walk/flash/E at them to cast your ult, they pull a Korean-level reflex dash as you cast it and now you're sad. I can almost guarantee this is what you're experiencing, I've seen it a million times.