The meniscus lenses in most box cameras, even when they haven't been reversed by a well-meaning "artist", have some field curvature -- which causes exactly what you describe: images that are fairly sharp in the center, but less and less so toward the corners (coma, spherical aberration, and chromatic aberration all contribute to this as well). For a camera originally intended to make contact prints from the relatively huge negatives, this wasn't a major drawback compared to being able to sell the camera for a couple bucks.
Unfortunately, all you can really do is make sure the lens is clean, correctly oriented (concave side toward the aperture stop, whether that's in front of the lens or behind), and whatever features the camera has to flatten the film (usually just a pair of rollers, the back of the box acting as a sort of pressure plate, and tension on the film) is operating as designed -- and accept that this is a camera that automatically emphasizes a centered subject.