One thing that comes to mind, is that there's often some sort of a "Fast Boot" type option in the BIOS. Others can explain it in greater detail, but the long and short of it is that if it's disabled the BIOS will do much more extensive checks of the RAM. This can take several minutes to hours, and there's not always any kind of visual feedback as to what's going on.
So, at least initially I would check for such a feature and make sure it is enabled. Also test with a bare minimum config, which includes the smallest amount of RAM needed to get the unit to POST.