https://download.asrock.com/Manual/Z77%20Extreme6.pdf
On page 14 you can see the PCI and PCIe slots
1.) Which one do you use for your GPU?
2.) Do you have any other PCIe cards installed which can cause the x16 lanes to be split up to x8 and x8 to the GPU and the other PCIe-card? Can be a PCIe-SSD, ...
3.) What GPU do you use?