One of my devices no longer shows up on my device listing

If you followed the instructions on how to prep your device before giving it away ( What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support ) then that would mean you had removed it from your Apple ID. At that point it no longer would show on your device list. If this is the case all you can do is file a claim with whoever you mailed it through. If they can’t locate the package they may have to pay you up to the amount of coverage the package had (assuming you paid for insurance or if the shipping method used offered any).