I have a similar problem however it's a bit strange.
I have an iMac mid 2011 i5
For a period of time I had 2 ram X 2GB and 2 ram X 4GB a total of 12MB
Recently the slots of the 2 rams of 2GB appear empty.
Ok... I bought another 2X4GB to replace them and to upgrade the total memory in 16GB
I put the new couple but again only the 2 already installed chips regognised (the down couple- bank 0 and 1 in Dimm 1) The upper part (Bank 0 and 1 in Dimm 0 appear empty)
First thought were that the ports had an issue as described in the thread.
Of course I have checked all chips individually and all are ok.
The strange part is that the upper slots WORK normally and recognise the chips IF the down slots are empty.
If ALL slots have a chip inside, only the down couple of chips (Bank 0 and 1 in Dimm1) is recognised.
So, neither the chips have an issue nor the slots...individually.
I will appreciate any idea!