You probably need to remove, i.e., clip away, the upper part or rim of the black connectors that plug into the motherboard connectors at the end of each heat sink. I had to push them out of their compartments before trimming the rim near the cables at the top of each connector. Freeing them from the compartment they're plugged into allows you to gently, manually plug them into the mother board connector completely just as you position each heat sink over the cpu's. Removing the rim prevents cable pinching and allows the connectors to be manually manipulated. After you've removed the rim remember that the black connectors aren't now ( and will never again be) secured to the heat sink ( other than by the tiny cables themselves) the next time you remove the heat sink; thus, you must gently remove the black connectors from the connector port on the mother board, using needle nose pliers, before you get the heat sink to far from home. When those black connectors weren't fully plugged in, mine wouldn't post either. But I figured out the problem - the IHS height difference on the cpu transplant with respect to an 8-core, causes less than a complete connection, evidently triggering a failsafe.