Hi, thanks for using JustAnswer.com. Please remember, I do not get paid for my time and expertise until you accept. Are the marks streaks or spots? Have you tried using any commercial machine cleaners? Do you use chlorine bleach?
Okay, you likely have a build up of mold and/or mildew throughout the machine, especially if you do not use chlorine bleach. You will likely see better results with using a commercial cleaner ATLEAST monthly. There is a stronger, industrial cleaner that works much better than anything you can purchase locally, but is sold in bulk only. However, it does a MUCH better job and will last for a couple years or more, even if used monthly, as I usually recommend. It is available at the link below.
Affresh Industrial Cleaner