make sure you have the latest drivers installed
https://community.amd.com/docs/DOC-1432#jive_content_id_What_Are_The_Requirements_To_Use_AMD_FreeSync
on monitor settings ensure the DisplayPort 1.2 channel is enabled and selected Enable AMD FreeSync™ in the Display OSD setting / on radeon settings navigate to the Display tab - Set AMD FreeSync™ to On / make sure you are running on fullscreen