You're doing all the right things. If these fail, then try doing the hottest possible cycle with a couple of dishwasher tablets thrown into the machine. This can damage the seal, but then so can mould. I would also check the filter again after and remove anything that cleaning the drum has shaken loose and flushed.
It might be time to broach the subject of getting the seal replaced with the owners. If the washer is under warranty that won't cost a cent but if not, then will need a repairman to call plus cost of parts.