Cass w not blocking mobility spells consistently

I've noticed that 's W doesnt always block mobility spells, i had a game where lee was regularly able to hop out of my w even though it was on the ground for a sec before he casted it, in another the enemy ww was able to ult out of it (again my w was already on the ground)

Its really annoying when you put it down to stop mobility but it doesnt do anything