Optiplex 790 Displayport issue

I have an Optiplex 790 and am trying to set up Dual Monitors.  Both Monitors have DVI and VGA inputs.  My 790 has a VGA and Displayport output.  I have the standard VGA cable and it works great on both Monitors.  I also have a Displayport to DVI adapter with a DVI cable.  When I hook this up to either Monitor get nothing.  The Monitor acts just like it does when it is not connected.  I've tested the Displayport adapter and cable on anther set up that is working fine so I know the cables work.  In the video settings it's only showing I have one monitor.  Any help would be great.  

Thanks

Chris