how do we fix this retained water in machne?

I have a Simpson 550 and has water left in bottom of machine when finished cycle ,it has been smelling .till next time i use then happens again.I use a cold wash cycle all times.I have put through epsom salts and left 24 hours still smells . Should this machine leave water in the bottom ,I wouldnt have thought so ,had other simpson machines never ha this problem. The clothes always come out spun dry ok so what could be the problem...