Upgrading the memory on Mac Pro 5.1

I currently have 2gb x 4 (8GB) in my quad core 2010 5.1. I bought 4gb x 4 and installed it and it only read 2gb per slot. I've check that it was seated correctly and try again still only reading 2gb per slot. I return this set thinking it's defective and the replacement memory is doing the same thing. Any ideas? How do I check for a problem with my machine? This kit is used Kingston memory.

• 4 x 4.0GB PC10600 DDR3 Kit
  
• Size: 4GB (4096MB)
• 240-pin SDRAM DIMM
• 512M x 72,...

Upgrading the memory on Mac Pro 5.1