Replacing drain boot/elbow #WD18X213

HELP!! How do I reinstall the drain boot or elbow (part #WE18X213) that goes from the back of the tub to pump? The old one just pulled out of the tub after I loosened the clamp at the pump. However, I cannot seem to get the new part to properly seat in the bottom of the tub. I can get it started in the opening in the tub and then it seems to push back out the bottom. HELP!

Pull out the dishwasher, lean on it’s back and put it on all the way around.