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
"4GB PCI Hole Granularity
This feature allows you to select the granularity of PCI hole for PCI slots. If MTRRs
are not enough, this option may be used to reduce MTRR occupation. The options
are: 256 MB , 512 MB, 1GB and 2GB."
The default is 256Mb. Set it to 2Gb (4Gb in some bioses) and the machine will boot.
This must be done before all the ram is installed, You can boot into the bios with a single stick installed.
same applies to the x7dbe+ and x7db8+ boards. (16 dimm slots)