Mac Mini (mid '11) won't accept RAM

My dad bought a used Mac Mini (mid '11) and it came with 2 x 1 GB RAM installed. We have tried to upgrade to 2 x 4 GB of RAM and here's the problem: First we bought Kingston RAM, using Mactracker as a guide, telling us to get 204-pin PC-10600 (1333 MHz) DDR3 SO-DIMM, which we did. No good - the Mac refused to start and we got beeps instead. We thought it had to be the RAM - installing the original RAM and the Mini runs fine again. We returned the Kingston RAM, got some new in return, only to find that they did the same: The Mini just beeps and cannot start with the Kingston RAM. Thus, we returned the RAM once again and then we found another manufacturer, i.e. Crucial. Low and behold, they do the exact same! The Mini will not recognize them, even if they switch places or if we try with one of the original + new, it fails.

We bought these Corsair: Corsair Mac Memory - DDR3 - 8 GB : 2 x 4 GB - SO DIMM 204-pin (CMSA8GX3M2A1333C9).

When we gave up and tried to install the original RAM, it beeped again! Switching the originals and it finally worked again.

Anyone who knows what is happening here??

Thanks in advance,

Dan