On past projects, I've seen a common theme. A particular feature will require very simple front end work (eg. make a button on a screen that says "submit"), but there will be a large amount of work on the backend. If we have say one backend developer and one front end developer, the backend dev finds themselves often doing work that doesn't get feedback, is behind the front end by several days or more, and so on.
Some solutions would be to either slice the story in a more even vertical between the two types of work, to go less "in-depth" with the backend work, or to work to make your developers cross-functional between the back and front end work, but I haven't had a situation yet where that's really a reasonable solution. Is there anything else that can be done?