we're sorry you are having issues with your 390. They should be compatible. The only difference is, the 390 doesnt support dvi-vga adapters and requires an 6+8pin power connector, instead of 2x6pin.
If you are sure those are hooked up right, either the motherboard doesnt recognize the 300 series, or the GPU is faulty. It should be that simple. Did you try the 390 in a different PC or any different cables or adapters?
Mark at XFX
completely uninstall the gpu drivers before installing the new gpu