Hi @Shuvajit Roy ,
There are many reasons for this situation, let’s check them out together:
1, The account running the User Profile Service Application does not have directory sync permission. This can prevent the profiles from being imported from AD. To fix this, you need to grant the directory sync permission to the account. You can find more details about this in SharePoint 2019 User Profile Service not syncing.
2, The synchronization database is involved in a database mirroring session or an availability group. This can cause the User Profile Synchronization service to fail to start. To fix this, you need to remove the synchronization database from the database mirroring session or the availability group. You can find more details about this in You can't start the User Profile Synchronization service.
3, The synchronization connection settings are incorrect or incomplete. This can cause the synchronization process to fail or not import all the profiles. To fix this, you need to check and update the synchronization connection settings, such as the domain name, the container, the authentication method, and the filter. You can find more details about this in Configure profile synchronization by using SharePoint Active Directory Import in SharePoint Server.
For your reference:
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If the problem persists, it is recommended to check the ULS log to see if there is relevant information.
Hope this is helpful.
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