Ive always used powder but recently my new housemate has said that the washing machine has started to smell and also it did at her last place.
I've tried to clean the machine - bleach wash, vinegar wash etc, leaving door and tray open after use.
She uses the washing machine quite often - a bag of Daz pods have lasted about 6 weeks and there are 52 in a pack - I've used a few. She will use two per load of wash and multiple loads per week.
I've had the machine three years. I've not come across a smelly machine. Maybe it's because it's built in?