upgrading 16gb to a 4gb imac i5 2011 doesn't work with all modules loaded

I don't know if it is only my issue. I have a 2011 iMac i5 with 2x2GB RAM modules. Recently I bought 2x8GB RAM modules. If I add the 2x8GB to the default 4GB RAM, the computer doesn't start. I hear only the sound of DVD slot but the screen remains blank. If I remove the 2x2GB ram modules and I use only the 2x8GB RAM modules, everything is fine and the iMac shows correctly the 16GB. I have also tried to change the order, putting the 8GB modules as first, but same problem.

If I put 2x2GB with the 2x8GB together, iMac doesn't turn on.

Original ram : 2x elpida 2gb pc3-10600s-9-10-f1 Upgraded ram: 2x hynix 8gb ddr3 dream 1333mhz so-dimm (solder as 'apple iMac 2011 compatible)