Hi, From what I've understood, you're testing the SSN presence in a doc/xls file using Winword/Excel. But do correct me if this isn't the case.
Presuming the above scenario, If the Label is published, and all your office clients across PCs/Users see the same behaviour of the label not applying, do review the Office versions which support Labelling. These Office versions are documented here https://learn.microsoft.com/en-us/purview/sensitivity-labels-office-apps#support-for-sensitivity-label-capabilities-in-apps.