Here are some possibilities:

• Sniffer isn't being launched successfully. It would show up in the log if it were and it is virtually incapable of running without producing a log file. It may be that the path to the program is unclear. Double check all paths to make sure none of them are relative or pointing to an incorrect location.
• Are log files turned on (mode='file') in your configuration file? If mode='none' or mode='api' then no log file will be created. 

      <!-- No classic log file -->

      <classic mode='none' rotate='yes' matches='unique'/>

      <!-- XML log file is turned on -->
               
      <xml mode='file' rotate='yes' matches='all' performance='yes' gbudb='yes'/>

• It is also possible that something on your system is preventing SNF from writing to the log file. Check your securities.
• You can also try launching it from the command line using exactly the same environment and syntax that your MTA will provide when it launches the program. This should provide you with an error message you can use.