Stainless is often just not clean, but it isn’t as obvious as glass when it’s not clean. I could see that making a difference. If you’ve got a stainless-lined mug or carafe, try pouring a bit of baking soda and water in to make a paste, and scrubbing a little bit. If you wipe it out with paper towel, you can see how much coffee stain you’ve missed cleaning out.