It's a memory/CPU problem.
Make sure they are seated in the 2 slots most close to the CPU.
If you use the most far slots, the CPU will downgrade the speed.
It also depends on the brand and type memory you use.
Try with one stick and see what happens.
Also, on some boards you need to set the mem-max-lock to the max speed in the list, as it could happen that your memory has DDR333 AND DDR400 SPD programming and it could happen that AUTO is taking the wrong settings.
Also it couls happen that your memory can't run higher with the AMD64.
Many options could be the cause, to find all info, look at the memory sticky on top.