Question Extra memory only sort of recognised on P9x79

[anynameyouwish]

I added a couple of sticks of 8Gb memory to my motherboard recently.
I already had 4 sticks of 4Gb in slots A1, B1, C1, and D1, and I added the two new sticks to B2 and D2 as the manual suggested, but instead of the total memory increasing by 16Gb, it only went up by 4Gb.
The BIOS manages to list the contents of each slot correctly, but also shows the Total Memory as 20480Mb which obviously doesn't add up to the sum of the slots.
Any ideas?

 

[rgd1101]

exact model of all the memory stick?

 

[anynameyouwish]

exact model of all the memory stick?

According to the BIOS:
The 4x4Gb are Kingston DDR3 1333MHz
The 2x8Gb are Kingston DDR3 1600MHz
Part nos 9905471-011.A00LF and 99U5471-054.A00LF respectively according to asus sysinfo.

 

[Bob.B]

I added a couple of sticks of 8Gb memory to my motherboard recently.
I already had 4 sticks of 4Gb in slots A1, B1, C1, and D1, and I added the two new sticks to B2 and D2 as the manual suggested, but instead of the total memory increasing by 16Gb, it only went up by 4Gb.
The BIOS manages to list the contents of each slot correctly, but also shows the Total Memory as 20480Mb which obviously doesn't add up to the sum of the slots.
Any ideas?

What happens if you use just the two 8GB sticks?