To use all four slots on this model, you need a quad-core CPU.  A dual core CPU will use only one bank (two slots), 8G/slot, 16 G max (with the quad core it's 4X8 or 32).

It probably shipped with a quad-core -- and somewhere along the line, someone replaced that with a dual-core.  

The four slots will work together only with a quad core CPU -- with a dual core, only two will work.

Thanks 

I know there is a 2 slot version, but I have the 4 slot version.

I don't understand why I would have 4 working slots but with only 2 at a time working.

Is this the same setup used for the quad core except mine only has a dual core?

Oh well, so I just need to either get 2x8gb and sell the 4x4gb ram cards OR swap out the processor for a quad core.

Thanks for the advice, much appreciated.