You don't need the USNO-B1 to plate solve at the focal length and FOV you're talking about. The USNO-A2.0 catalogue will meet the criteria with no problems at all. It contains 526,280,881 stars down to Mag 20. If you can't use this, there is something wrong with your set up / software configuration. You can download it here
ftp://ftp.nofs.navy.mil/usnoa - 6Gb download.
Keep in mind for successful plate solving you don't need to solve every single star in the field. Typically 20-80 stars works well. The more the better, but it can slow the process down. Finally, make sure you telescope is pointing reasonably accurate. The plate solve process looks at the RA/DEC in the FITS header to pass to the plate solve process. If this is significantly out, plate solving will fail and/or take a long time solve as it churns through the lookup catalogues.