I am having the exact same issue and I've never had PUG's installed.
Here's a picture of the bad side:
Here's how the canvas sags:
A picture of the good side (although it looks like it might be out soon):
There are 2 three inch pieces of plastic U-channel in each corner. Not quite sure what they are supposed to do. I took them out from the outside so that they would get lost. I don't know whether to take it in for repair or try and fix it myself. It is out of warranty so I'll have to pay one way or another.