My washing machine smells sour and I don't know how to cure it.

My clothes and the washer itself smells sour and like stagnant water. I tried the stuff you buy at the store and it did not help. I also tried cleaning it but the smell comes right back after one day. I am not ready to purchase another one but I am almost to that point. I would love an answer for the person who has not yet purchased a front end loader. I have a top loader agitator kenmore/whirlpool machine.