It sounds like you may have lint or some object blocking your drainage hose that goes into the sewer line. You can remove it from the end that goes into the wall to inspect it, or you may be able to disconnect it from the back of the machine to see if it's partially clogged. If that drainage line is clear all the way through, then your problem is in the sewer line.
There should be a cleanout cap on the sewer line, where you can run a snake- line in there. There may be fabric fibers and/or a grease-like build up in there, prohibiting full drainage of the water from the washer tub!