Ours popped off a few years ago. I had to go in the attic to put it back on. Obviously, I can't see what you have or what's working or not for you but our stairs needed to be closed in order to do this. If you're trying this w/the door in the DOWN position, it's no wonder you're having troubles.
My best to ya and hope this helps.
PS: Make sure someone is 'down below' when you close the door. Should you have a problem or want to come down, you'll want someone around.