PC fullscreen issues

My game launches in a square 2/3 the size of my screen but my options insist fullscreen is enabled,. Changing resolutions does not change this, in windowed mode I can change the size but I want it running in fullscreen

Any ideas

To confirm you are using the correct display size ratio for your monitor correct? For most standard widescreen monitors, you need to set it to 16:9 at 1920x1080.