Hey, Barry.

What kind of 'flexing' are we talking about (does the top bow in or is it spreading out)?

My first take on this is that the liner might be a little too snug (possible reasons, liner cut too small, liner put in on a 'cold' & cloudy day, Bottom not at proper level - too deep, or the cove being absent or too small or any combination of these)

Please get back with me on this and we'll try to figure out the best fix, if there is one.