Active Directory user groups not syncing

I am using Azure Active Directory Connect to sync to Office 365. I am syncing users and groups however I am having a problem with syncing user groups. Basically I am adding new users to a local security group those new users are never reflected in Office 365. To be clear my local user group has 100 users the one in office 365 will show 50 users its like the changes never sync to office 365. Also I created a brand new user group as a test case that user group never synced to Office 365. So my question is how do I properly troubleshoot this?