it's kind of hard for the X58 platform running at 2133Mhz, please try channel A/B/C with 1866Mhz and see if the board still reporting 8GB or not, if the system show correct 12GB, which is mean your board could not supporting 2133 as well, please note!
here is the memory SPEC of your motherboard:
6 x 1.5V DDR3 DIMM sockets supporting up to 24 GB of system memory (Note 1)
Dual/3 channel memory architecture
Support for DDR3 2100+/1333/1066/800 MHz memory modules
GIGABYTE - Motherboard - Socket 1366 - GA-EX58-EXTREME (rev. 1.0)
but at 2133Mhz, it may not fully supported as well. thank you!