Shaco box fails to stop blitz grab or thresh grab when box can't see enemy champ

I have been playing shaco as support. I start with lots of AP and control the bushes. If blitz is in the lower bush and I place a box in the next bush the box should be there to stop any grabs that attempt to go through the box. What happens is the grab goes right through the box and gets me. If I am out in the open the box will usually block the grab, but is not very consistent probably because only the center of the jib seems to block the grab instead of a hit box the size of the jib.