Have you crawled under there and looked at the posts? I did this and found a small block of wood
on one post, kinda looks like it has been taped on. I need to investigate further but I'm thinking that this post was mounted too low and it would "slip"on full port over. Not sure why else one side would have this attached and the other does not. If nothing is blocking it, then sorry, I've got no other idea for ya except this one personal example.