Directory synchronization is a key component with Office 365, Password Sync is one of the key features of Directory synchronization that is implemented on a vast scale, with recent deployments by both Organizations that deploy AD FS for SSO and also by Organizations that rely on Password Sync for Same Sign-on.
This article contains information to help you troubleshoot common issues that you may encounter when you synchronize passwords from the on-premises environment to Azure Active Directory (Azure AD) by using an Azure AD sync appliance.
Access the Troubleshooter here: http://aka.ms/hrcsync
Access the complete Article here: How to troubleshoot password synchronization when using an Azure AD sync appliance
Read the first section "Before you start troubleshooting" and ensure that you have the latest version of Azure AD Connect installed.
Note: All other Azure AD Sync appliances are being deprecated. Therefore, if you're using another appliance, install Azure AD Connect.