That bed liner in the dump bed might have been a bad move. All your debris is going to stick to it and not slide out.
That and it's a waste of time if you haul a lot of material. I just keep the sides painted up in the bed and throw some cheap graphite paint on the floor before winter to keep my salt sliding out effectively.
Sometimes I just wait for a sheen of rust to start on the floor of the dump and put a rust converter down. That stuff is pretty slick also.