New Liner installed wrong?

So first a little background. I moved into our house about three years ago. It has an in ground pool. The previous owner left us a note about the pool. He said there is a lot of ground water in the back yard. This causes a lot of water to accumulate behind the liner. To combat it, there is a return line set up in the shallow end behind the liner to pump the water out. So after a few days of rainfall the liner would seem to float and be bouncy in the shallow end. So I turn on the line behind the liner and pump it out. After pumping it out, the liner would be nice and tight and snug all around the floor and walls. I would do this about once a month.

So last week, my old liner was brittle and cracking. So I got it replaced. Immediately, I noticed the where the walls and the floor meet in the shallow end,its bouncy and trampoline like. It's not flush and hard against the wall/floor. So I call the company that installed the liner. They insist that it's done properly but it's because there is water behind the liner. I told them I tried to pump out water from behind it but there is none.

Is it possible the liner wasn't measured properly or some other cause? There definitely isn't any water accumulation.