Optiplex 960 and missing RAM

I have purchased an Optiplex 960 Tower from a Dell Partner with 8Gb of RAM, or that is what I have paid for. Wherever I find System Information on the computer I am being told that there is only 6Gb of RAM. So what has happened?  I have taken this up with the Dell Partner and they tell me that there should be 8Gb of RAM in the machine but they will look into this and come back to me.  All has gone quiet on this one and before I start chasing I wonder if anyone has any ideas please?