I'm a writer so I know the relationship you're talking about pretty well. It's difficult! The copywriter has certain goals and the designer has certain goals. Of course their shared goal is a good final product, but along the way I guarantee that there will be disagreements over how to get there.
Have you tried setting up a meeting just to discuss how the two of you work together? I do frequent check ins with the designers I work with to help me understand their working styles better. And that makes it a lot easier when conflict does arise... if it's already been discussed, it's a lot easier to laugh about it and smooth it over.