Dell poweredge R620 RAM Population

In my Dell poweredge R620 server i install 8GB 12 Memory stick in slot B1-B12,but some memory stick not showing(B5,B6,B9,B10),server usable memory is 64gb now.Plz suggest me RAM population.

Edit:All RAM are same configuratin(8gb,2Rx4,DDR3 PC3L-10600R 1333MHZ) OS-Windows Server 2012 Standard