Strange Lenovo X200 memory problems

I recently obtained a used Lenovo X200 Thinkpad. It came with 2GB of memory, which I wanted to increase to 4GB ie 2 x 2GB modules which is what the manufacturer states is the maximum. Unfortunately these are difficult to obtain, and according to some commentators the X200 will run 8GB. I purchased 2 x 4GB Corsair and found that one module would function in slot 0, whilst the other module would not function at all. I updated the BIOS to the latest firmware and was then able to run 5GB, which meant that the 4GB limit no longer applied.

I took the memory back to the supplier, Inet on Ringö, together with the computer. We tried another pair with the same result,. Then we tried single 8GB modules which did not work. Finally we tried a pair of AData 4GB modules which worked without a problem, so I have used those. First class service from the Inet technician. But it seems as if there is a significant different between memories from different manufacturers, though nominally of the same specification, as well as different degrees of consistency.