Any chance you are running Mojave with an APFS drive? If so, I have had the same problem. BootCamp Utility does not seem to like seeing the boot drive as an APFS drive. When I was fighting with it, I had to reformat the Bootcamp into APFS and eventually get each partition to merge into one. I strongly suggest that if you take this path, you back up all crucial data, and do not play risky like me!