It should support 16 GB, and even ignoring that, it should be reporting slots as being filled.
Take everything out but 2 chip and then keep exchanging them until you've checked them all. If you find any bad RAM, well, that's the answer. If all the RAM is good, it's time to call Apple and ask them, in that case it's a problem with the computer.
I did actually sort this out I did'nt figure out how the slots were numbered and i had conflicting pairs in slots. Now running at 14GB (6x2GB & 2x1GB) Have'nt tried to render anything yet but it must help.