Many systems require memory to be installed in pairs, but each pair are not in adjacent slots, but interleaved or in separate "banks". You need to read your hardware documentation to determine that, but I expect that this may be the cause of the "missing" memory, especially after you last "experiment" which gave you another 2gb of RAM, yet did not provide all that you have installed in the system.
My MSI MB uses alternating slots for the memory and I have always installed them correctly, according to the manual. I have been building my own computers for over 20 years, so I'm pretty sure I didn't screw that one up.
Originally Posted by Rubberman
Ok. Just checking - due diligence and all that cruft! :-) In any case, no matter our experience, sometimes we have a brain fart and just forget to check the obvious... I know it happens to me (witness the "bricked" laptop on the second day of my new job).