Check the motherboard's manual, the chipset might not support over 16Gb RAM. I currently have 4gb in my board, I can have 8gb, but my chipset supports 16gb. My friend has 8gb in his, he can go to 16gb, but the chipset supports 12gb.
In short, maybe your board has a restriction on the maximum supported RAM. Also, it could be a maximum amount of banks that it supports, ie: If you have 4x 2 sided sticks (8 banks), and your board supports a total of 4 banks, then your other 4 banks are useless and you can't utilize those.
I understand Supermicro boards are somewhat known for being fastidious with which RAM sticks they'll work. On the bright side, there's a compatibility list on their website so that you can verify which RAM sticks will.
"The X7DB8/X7DBE supports up to 16 GB (32 GB) fully buffered (FBD) ECC DDR2 533/667 in 8 DIMMs. Populating DIMM modules with pairs of memory modules of the same size and same type will result in Interleaved Memory which will increase memory performance."
Q - are the sticks ALL the same Mfg, Speed 533 MHz and 667MHz, Freq, Fully Buffered ECC, etc...? If not then note the 16GB above