Check the image resolution. A number of covers on this site were created using pixel dimensions, which makes no sense for anything intended for print.
What you’re looking for is an image that is 72dpi and has massive measured dimensions (as in WAY over 8.5”x11”). If you find that to be the case, then simply adjust the resolution to 300dpi without resampling it. This will bring the size back to what it should be for print.