Fortigate 90D AD Integration configuration

Is there a fail safe method or process to implement AD integration with FG90D's? We have implemented AD integration for Web Filtering and Monitoring on a number of FG90D devices but have been having endless issues with getting it to pull the usernames.

We have installed in every possible way but still are having issues. So I hope that there is a blueprint to follow.

The DC's are Windows Server 2012R2 and we have tested the FG90D's using all of the last 6 Firmware versions. It was working for a period of time on a couple of the sites but then stopped working without any changes to the environment.

I have spent a huge amount of time trying to troubleshoot but have not been able to resolve.