PC doesnt post unless I relocate my GPU

[minikord]

I recently bought a new Mobo, CPU and RAM. After installing everything correctly my PC posted as usual.
After that I did a reinstall of Windows.
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I later turned off my computer after setting everything up.
The next day my PC didnt post. After some troubleshooting I found out that my PC would post if I moved my GPU one PCI-E slot down. I thought everything was fine until the next morning when I had the same problem, I then tried to move the GPU one PCI-E slot up and that worked too.
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I've tried all I know without sending equipment back to the retailer.
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Any tips?
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PC part list:
Mobo: ASUS ROG Strix Z270F Gaming, Socket-1151
GPU: MSI GeForce GTX 1080 Armor OC
RAM: Corsair Vengeance LPX DDR4 3000MHz 16GB
CPU: Intel Core i7-7700K Kaby Lake Prosessor
Storage: Corsair Force MP500 240GB M.2 PCIe SSD
PSU: Corsair RM750x 750W PSU
OS: Windows 10

 

[rocktalkrock]

Look in Control Panel > Device Manager and make sure there are no yellow warning signs and all your drivers are up to date. Look closely at "System devices".