Are you running with SP4 on the server?
What features are included on your deployment? Only the antivirus and antispyware components should be installed. Do not install Network Threat Protection and/or Proactive Threat Protection.
Are you seeing any errors?
For most installation issues, you can find the installation log (located in the 'Temp' directory) for detailed errors/events.
BTW, I am moving your thread to the SEP 11/AV forum for better visibility.