The easiest way to address this issue is to use the troubleshooter tool available here: Azure AD Troubleshooter. Alternatively, you can follow a more manual approach by following these steps: Manual Troubleshooting Steps.
Verify is the user is in an OU that is in the scope of the Azure AD Connect (begin by checking the object properties in the Metaverse).
Look at the connectors tab and ensure that at least two connectors are listed. Verify the join type. If there are not two connectors listed, the object is likely filtered.