Sometimes, the serial number field may not be mapped correctly, This issue is happened to me either: take a quick look, hope you can find easy way buddy:

Go to Discovery → CI Classification → Classifiers.

Search for Windows Server.

Open the classifier and check whether Serial Number is mapped correctly.

Check Identification Rule?

Navigate to Configuration → CI Class Manager.

Search for Windows Server

Open the Identification Rule.

Ensure that the Serial Number is properly included in the Identification & Reconciliation Rule (IRM).