Had a new quadrant shower put in a few months back. It has started to dip and as a result water is pooling and not draining away in the middle. Victorian plumbing (tray supplier) and the builder are both refusing to take blame. I finally hired an independent contractor today to break a few tiles at the bottom so we could see underneath. See pic attached. To me it looks like no adequate timber frame and the independent contractor agreed. Who is at wrong here?