I understand that you’re having trouble syncing some users from your on-premises Active Directory (AD) to Azure AD. Here are a few suggestions that might help:
Check the Azure AD Connect Health: Azure AD Connect Health can help you monitor and gain insights into your on-premises identity infrastructure. It can help you maintain a reliable connection to Office 365 and Microsoft 365.
Force a Synchronization: You can force a synchronization between on-premise Active Directory (AD) to Azure Active Directory (Azure AD), by using the Start-AdSyncSchedule PowerShell cmdlet from the ADSync PowerShell module.
Troubleshoot with Azure AD Connect: Open a new Windows PowerShell session on your Azure AD Connect server with the Run as Administrator option. Run Set-ExecutionPolicy RemoteSigned or Set-ExecutionPolicy Unrestricted. Start the Azure AD Connect wizard. Navigate to the Additional Tasks page, select Troubleshoot, and click Next.
Check User Attributes: Ensure that the users you’re trying to sync have the necessary attributes filled out, such as userPrincipalName and mail. Also, make sure that the userPrincipalName matches the user’s email address.
Check Synced OU: Make sure that the Organizational Unit (OU) where the users reside is selected for synchronization in Azure AD Connect.
Check for Filtering: Check if there’s any filtering applied that might be preventing these users from syncing.