I’ve encountered a situation where computer with 8Gb RAM installed could only use 4Gb of it. BIOS recognized 4Gb of it and Windows told that there is 8Gb installed of which 4Gb is usable and the rest is “Hardware reserved”.
After some browsing through forums, I found an advice to swap the modules around in memory slots until full amount is recognized. Believe it or not, this actually worked. After some more digging I found a claim that the memory modules would be configured to work either on bank 0 or bank 1 on dual channel. I know that the modules are most of the time quite finicky about being installed in to the same bank if purchased as e.g. 4Gb kit of 2x2Gb modules, but is it actually true that the modules would be configured to work only on either channel? I couldn’t find a confirmation for this theory.
