The RROD X-Clamp fix did not work, why?

MY friends Xbox 360 had the dreaded RROD recently and since I fixed a few prior I figured no sweat, but unfortunately this one was not fixed. (Actually, my luck with fixing Xbox's is at a current 2-2. Two worked and two didn't.)

Im now left wondering why this time the Xbox didn't work. I used the "X-Clamp" fix where you replace the X-Clamps with 5mm machine screws and applied new thermal paste (even using the thermal paste remover and purifier.) I feel that im doing everything right, am I missing a step? I'd love some help or pointers!

Thanks in advance.