Hello, that is strange, your desired configuration should work as it respects the memory installation guidelines. It could be an issue in one of the slot 7,8,9 in the motherboard, I don't have other explainations. Thanks Marco
My Dell R710 is on latest BIOS 6.6.0 and had an existing DDR3 RAM of 72GB with slot configuration as below Current Config(72GB): 8GB in Slots (1,2 &3) + 4GB in Slots (4,5&6) on both sides giving me 72GB. but when i add additional 4GB in slots (7,8&9 on both sides) ( Total 4*6GB) system do not boot up. Desired Config(96GB) 8GB in Slots (1,2 &3) + 4GB in Slots (4,5&6) + 4GB in Slots (7,8 &9) : system do not boot up. BIOS config is in optimizer mode. All memory modules (4GB & 8GB) are RDIMM and Dual bank & work well in slots 1-6. I read a lot but could not find a possible reason as to why things are not booting up in slot 7,8 &9. Any help will be appreciated   
Dell Power Edge R710 RAM Upgrade issue 72GB (6x8GB+6x4GB) to 96 GB 
(6x8GB+6x4GB+6x4GB)