Could just be a compatibility thing?
*quick google*
Is the RAM double sided? according to this thread on whirlpool the motherboard will only support 8GB over 4 banks (i.e. 4 single-sided DIMMs)...
Checked and confirmed on ASUS website:**Due to chipset limitation, this motherboard only supports up to the total of 4 ranks (4 sides in total) of RAM at max.