Ensure that the Xvfb X server and xauth for your Linux distribution is installed. This is included with most Linux distributions, but not installed by default. You won't get anything in your PDF report unless this part is setup on the server.
On Redhat/CentOS/Fedora, type: yum install Xvfb xauth (note the capital X for Xvfb.)
On Debian/Ubuntu, type: apt-get install xvfb xauth