You can get “bands,” or horizontal lines across your print for many reasons. Typically, it means that something is loose. It’s not always obvious where to start looking, though. I usually look at the roller bearings first. Then I make sure all the nuts and bolts are tight. Then I check the belt tension. Just keep going through your machine until you find something that wiggles more than it should and tighten it. It’s not a glamorous process, but you should be going through your machine periodically anyway.