Welcome to the forum.
Is that valve that is leaking on top of the filter? Is it a sand filter? If so then it is what is called a multi-port valve. Just exactly where is it leaking? Could be any number of leak points so answering the questions will let us give specific advice. As for the liner, anything like you say should not be happening. Maybe 10 years or so later but not after a couple months. Keep after the installer. Keep records of phone calls and any correspondence.
Al