Popup Camper Top - RR corner won't completely lower / close

Lift it completely back up and start over. As you slowly bring it down, keep stopping and keep checking for obstructions. Take your time. Something is not lining up right and you are going to have to determine what it is.
Edit - What's in the rear corner? A sofa? Are you sure that it's as far down as it goes? I once had a sofa not collapsed correctly and had this issue.