Firstly, you will need to be on the machine that runs your syncronisation service. You will now need to follow these steps to run the troubleshooter that should hopefully fix your password hash sync issue.
1.) Run “Windows Powershell” as administrator.
2.) Run the following command
If that code does not work, you can try this one (but only use one or the other)
3.) Start the Azure AD Connect Wizard.
4.) Navigate to the “Additional Tasks” page and select “Troubleshoot” and click “Next”.
5.) On the troubleshoot page, click on “Launch” to start troubleshooting.
6.) In the menu, select “Troubleshoot Password Hash Synchronization”
7.) In the sub-menu, select “Password Hash Synchronization does not work at all” and follow the on-screen steps.