Q: 12GB limit?

I have a iMac late 2009 originally supplied with 4GB RAM.  After having Aperture slow to a crawl with regular spinning balls I thought a memory increase was needed.  I bought 2 x 4GB Kingston sodimms and this gave me 12 GB.  Great.  Then I thought RAM was cheap, I might as well go the whole 16 GB so I put 2 more of the same sodimms in and got only the S.O.S. beep code.  Swap the RAM around in the 4 slots - the same beeps - I return the original 4GB and boot up no problem - I put in 2 x  4GB  - no problem.  3 x 4 GB (any combination of the new sodimms) - no problem,  2 x 4GB & 2 x 2GB  - no problem - 3 x 4GB & 1 x 2GB - beep , 4 x 4GB - beep.     I tried each sodimm in a laptop and they all work fine and I cannot believe I have been supplied with 4 faulty sodimms in a row.   Seems that any combination I try that exceeds 12GB and the computer beeps. Now I am running with 2 x 4GB and 2 x 2GB and everything is fine, But the computer should take 16GB. Anybody seen this before?