After opening the pool this year, we noticed that the liner seems to have stretched maybe or something that one about ¼ of the pool had air behind it above the water line. below the liner is sucked nice and tight, as it is on the ground. Any ideas what might be causing this, if there's any concern, and any steps to correct it? The walls all seem straight still, and the posts are still level, so I'm kind of scratching my head as to what happened over the winter!

