My Optiplex 7020 doesn't detect my GPU!

Hello everyone, So I have Dell optiplex 7020 with an i5 4590 that I use for gaming but I am having a problem with it, It's problem is that in 3 out of each 5 boots it doesn't detect my GPU (which is an XFX Radeon RX 570 4 GB), When it doesn't detect it, it just boots off the iGPU (Which is an intel HD 4600), My RX 570's fans start running and it's blue lamp to the side (the lamp that shows if it's on) also gets lit when it's not detected as if it's working normally, I don't know why this problem is happening, but I am tired of it!

I tried updating the BIOS, I updated it from Dell 7020 A04 to Dell 7020 A18 and the problem is still there... The PSU I use is FSP 500 60APN which is a 500w PSU with 408w on the 12v rail... So the power supply is not what is causing this problem...

And help would be appreciated.