Sometimes the device list fails to refresh due to cached data. Try Chrome, Edge, or Firefox in private/incognito mode.
Make sure you're signed into the exact account used during registration. If you have multiple Microsoft accounts, try switching between them.
Go to support.microsoft.com and start a chat or call. A support agent can look up your device by serial number directly on their end and manually open a warranty case for you — they are not limited by the portal glitch.