have you double checked to be sure it isn't already onboarded. I know this sounds silly, but I've done this a few times. If there are any glitches during the onboard process and you don't get to finish out the steps, it will still have onboarded the device but it won't be assigned anywhere. Try global search to find the serial or mac, and then also try removing all filters so you have all APs in your monitor window, and search locally there.
Another possibility is that someone else has onboarded that serial or a database bug. If that's the case you will need to open a support ticket to have it resolved.
