The user manual clearly states that it supports 8GB modules for a total of 32GB. It also states it will support up to 64GB if 16GB DDR3 modules ever become available, which we know they probably won't, but they might with the upcoming release of Broadwell looming. Pretty doubtful though after DDR4 has already hit the market and resources are being utilized to advance that technology rather than worry about a module platform that will be relatively dead in two years time.
I think it's more likely to be an issue with the slot placement or a BIOS setting. I'd try one module at a time in the A2 slot, which is the yellow slot closest to the cpu. That is the slot recommended for single module operation.