How do I config XMP RAM on P8Z68-V GEN3?

Honestly you might have to mess with the timings of the RAM. When my RAM wasn't timed properly, my Sabertooth 990FX Rev.1 and AMD 8350 were failing to detect all of mine.

Also make sure that if there are pairs of memory, that they go in the correct slots. Like, say you have 2 white and 2 brown slots. One "pair" will go in either the 2 whites, or the 2 browns, but never a white and a brown.

graphuto said:

Honestly you might have to mess with the timings of the RAM. When my RAM wasn't timed properly, my Sabertooth 990FX Rev.1 and AMD 8350 were failing to detect all of mine.

Manually change the timing? That's not harmful? And how do I know to what?

The correct timings should be written on the physical memory stick itself.
Something similar to 9-9-9-24
Find the corresponding values in BIOS under the DRAM Timing Settings.

graphuto said:

The correct timings should be written on the physical memory stick itself.
Something similar to 9-9-9-24
Find the corresponding values in BIOS under the DRAM Timing Settings.

The timing that's on the RAM is already set automatically on the BIOS

Do you have a really high FSB clock? I found that if I went too high on my FSB clock, that it started dropping available RAM.

Could it be because the BIOS has never been updated? It's on 2011

The latest BIOS for that board was rev. 3802 on 1/15/2015 and the max the board ever supported was 32 GB. ASUS put out a 8GB QVL for the board which only included two chipsets back in 2011 to bring you to the 32 GB max for the board and those were A-DATA SU3U1333W8G9-B and SAMSUNG M378B1G73AH0-CH9 respectively. Anything done after BIOS 3802 came out never made it to the QVL as it had never been updated beyond 2011. You could try to update the BIOS to the newer version (buyer beware) and see if it will take.