I had a similar problem on first setting up my UD5 / 920 system. The BIOS was reading 4096MB with all 3 OCZ 12800 modules installed. I read these threads and some similar ones on other forums with a bit of a sinking feeling and thought I'd need to take it all to pieces again to figure out the problem.
However somehow I fixed it, I know this won't work for everyone but it's worth a try. I took out all the memory, and reseated the first module in DDR3_1 as recommended in the manual. The system booted (this is the point I understand some people can't reach) and in the BIOS the memory read 2048. I switched off and put the next module in, making sure to use enough force that the levers clicked in on their own. On booting I had 4096. All good so far. Switched off again and put the last stick in, and on reboot I had 6144!
No idea whether I hadn't inserted the memory correctly the first time or if flexing the board fixed a dodgy connection or if that sequence of events kicked the BIOS into recognising the memory but for now it's working fine. Quite relieved... best of luck if you have/get the same problem.
However somehow I fixed it, I know this won't work for everyone but it's worth a try. I took out all the memory, and reseated the first module in DDR3_1 as recommended in the manual. The system booted (this is the point I understand some people can't reach) and in the BIOS the memory read 2048. I switched off and put the next module in, making sure to use enough force that the levers clicked in on their own. On booting I had 4096. All good so far. Switched off again and put the last stick in, and on reboot I had 6144!
No idea whether I hadn't inserted the memory correctly the first time or if flexing the board fixed a dodgy connection or if that sequence of events kicked the BIOS into recognising the memory but for now it's working fine. Quite relieved... best of luck if you have/get the same problem.