You have to realize that whitetails are homebodies.When they are traveling in their core area,those places they travel everyday,they know every tree,every stump,every bush.If a new "haystack" appears next to their trail,you are going to see a whitetail turn inside out.And they won't forget the experience.You will have to move your ambush site and it's appearence before you have a prayers chance at those same deer.
I don't know if the spot allows it but,as others have said,I think if you were backed up to a big tree,blowdown or something else you could blend into,the ghillie suit could have done it's job.I think you added an obvious change to the terain that they knew shouldn't be there.
Go look at Big Jim's video on the "Bush in a Bag".As big as that guy is,look how he becomes part of that tree.He didn't sit just anywhere.He found an object big enough to blend into.
Whitetail does spend every waking moment trying to keep themselves and their fawns alive.They are probably the toughest big game in North America to take,once they get a few years experience.Don't take them for granted.The ghillie suit is an amazing thing but you still have to work it into the landscape.
Imagine if you came home from a long vacation.You open the front door and there in the living room is a pile of dirty clothes and muddy,size 15 boots-none of it was there when you left.Muddy tracks going down the hallway.What is your first reaction?An adrenaline spike,your hairs stand on end and you back the heck out of their.
That's what happened to those deer.