My first thought is the Linux* operating system is not working or detecting all the cores on the processor (might be a limitation) or the processor is defective (I highly doubt it). The best way to determine if is an OS limitation or a hardware problem is boot to the BIOS screen and check the number of cores listed (it will depend of the BIOS use by the board manufacturer).
There is other option:
Use the Intel processor ID utility (certify toll) on Windows. It will tell you all the processor information including the number of cores. There is not Linux* version, but you could try a bootable version, that may work. You can get the information and download from the following link: http://downloadcenter.intel.com/SearchResult.aspx?lang=eng&ProductFamily=Processors&ProductLine=Processor+Utilities&ProductProduct=Intel%C2%AE+Processor+Identification+Utility