Network Monitor does not show traffic on RHEL x64 for DLP version 10 and older.
Even though tcpdump shows traffic on the eth# that you have configured your Network Monitor to look at you do not see traffic in system overview for your Network Monitor, or the Traffic screen.
Symantec DLP does not support RHEL x64 on version 10 and under.
To make packetcapture work you will need to reinstall the OS with 32 bit Linux / RHEL then reinstall Symantec DLP and configure as Network Monitor.
To find out if you are running 32 bit or 64 bit RHEL you can run the following command: uname -a
If you see x86_64 then you are running a unsupported OS
If you see i386 then you are running a supported OS and have a different problem which you may need to contact support for and provide your packetcapture and filereader logs.