As said, verify that it's the PCIe cable.
I'm not sure which model you have, but the "TS" version of the XFX 550W has:
1x6-pin, and 1x6+2-pin
If you have those then they should fit fine. The EPS 4+4 pin should go to the motherboard near the CPU and the above PCIe cables should connect to your card.
*If you are using the correct cables then perhaps your "6+2" cable isn't correctly lined up.
**The connectors are almost "idiot proof" as they have notches in them that only align one way.