2 things to do,do a hot wash 90oC once a month,if u got a dishwasher,chuck 2 tablets init and run a 60oC wash and use soda crystals there cheap and work well!
next thing is to check the drain hose,does it go to ur sink?if it doing straight down to the washer?if it is chances are wen u put water down ya sink,its back flushing into ur washer,