I play this game called Darkfall Online which is a DX9 game. The refresh rate works fine for Nvidia cards (I tried with my old 470) but it just wont work no matter what I do on my AMD gpu. I tried with both my 7990 and a friends R9 270x. I had the latest version of the crimson drivers so I reverted back to 15.11.1.
I tried unplugging my second screen.
I also tried CRU to no effect.
Are you running it in Fullscreen or Borderless?
If it's borderless, then it should be forced to run on the same refresh rate as desktop. If it doesn't run the 144hz, then you probably have the monitor set to a wrong refresh rate.
When you go to your Display adapter properties, Monitor tab, it should be set to 144hz.
Do you guys have it set to the respective refresh rate (75/120/144hz) in Windows?
It needs to be set to the correct refresh rate there first.
Display adapter settings, Monitor tab.