Have you tried changing the speed of the RAM to 1333MHz and adding a little bit of voltage to them? If rated at 1.5V try 1.52 for example. Don't go to high on the voltage though, trying to get it to BOOT/POST if it won't fire up and you have the voltage set at 1.55V I would stop rather than keep increasing it. That said, it should be safe up to 1.65V but of course, more voltage increases the heat created and will shorten the life expectancy of the modules. Make sure you have a good airflow over the RAM sticks.
You could also try changing the timing settings a little. If they are rated at 9,9,9,24T try 9,9,9,27T or 10,10,10,27T to see if you can get it to BOOT/POST with all slots populated.