Welcome to the ROG Forum! And sorry to have to start this way...
Although, sometimes the refurbished products can be even better as the new ones, I understand what you're saying...
To test and be sure that one of the DIMM slots is not working, you can do two things (if you haven't done yet):
1. Get one memory module and insert in every slots while doing memtest86+ (do it on stock BIOS settings); link:
http://rog.asus.com/forum/showthread.php?19615-Memtest-User-Guide-For-ROG-Motherboards&country=&stat... Eventually, you can repeat this with every modules...
2. As the memory controller is located in the CPU, sometimes is worth checking the socket pins and reseating the CPU...
If none of these helps you, then I think you need the RMA process... Not sure in which country you are, but it may be worth trying first with your retailer, from where you've got the board...