A few years ago, our brand new liner was leaking at the same rate as yours. I had the company who installed it come and do a leak test. After at least 2 hours they found it. A pin size opening where the side wall was fused to meet the bottom. It did not overlap enough in that one spot. After a long battle with the installer, up to the owner, with lots of pictures and explanations, They finally agreed and replaced the liner. It did take 2 years and several attempts to patch the hole.