What do you clean it with?

Have you used cleaning products containing bleach and/or vinegar (acids)? If so, you need to reverse the chemical reaction which caused the staining by using a base. One way to do that is by using a paste of baking soda and water (I use distilled). Before you try this, however, make sure this is regular stainless steel, i.e., make sure it’s not smudge-proof or fingerprint-resistant. And always test in an inconspicuous spot first!