First, it's never a good idea to mix-match RAM, next 2x4GB + 1x4GB kills Dual Channel. Use the first (3) DIMM slots. Together, they cannot run DDR3-1600 and you're stuck to DDR3-1333 CAS 9-9-9-24-2T @ 1.50v
Q - What MOBO?
Memory Remapping -> Enabled (required for >4GB)
Keep the DDR3-1600 in SAME colored DIMM slots
Place the DDR3-1333 in OTHER colored DIMM slot but closet to the CPU
Typical problems include setting Maximum Memory; set MSCONFIG as shown: