If things aren’t going well with your (Ubuntu Natty 11.04) ntop, here are some things to try:

• Configure ntop to NOT run as a daemon. Stop the ntop service ("service ntop stop"). Run ntop in debug mode from the command line ("ntop -K").
• If your ntop log contains "NETFLOW: no devices to initialize", you might be having an RRD problem. Check your log for "RRD: Disabled - unable to create base directory (err 13, /var/lib/ntop/rrd)". Sometimes when you install ntop, the permissions/owner on /var/lib/ntop/rrd don't get set right. Kill ntop, "rm -rf /var/lib/ntop/rrd" and start it again.
• If your "NetFlow Device Configuration" shows an empty list of Available Netflow Devices, check to see if you're having an RRD problem. Fixing RRD fixed this issue for me. </ul>