Have you checked your filter,ie,cleaned it.If the filter is partially blocked the water empties in a slugish way and the water does not fully empty from the drum,although the machine looks empty some water is left lying in the bottom of the sump.
Or has the grey empting pipe become disloged and lying behind the washing m/c,it should be higher than the discharge entry into the sink outlet because everytime you empty dirty water from your sink basin it would flush some of this dirty water into your washing m/c sump.