A sniffer utility (such as Dagsnap, Wireshark, etc.) is capturing traffic but no traffic is being reported through the Monitor UI.
Check the IP Filters. The filtering could be set up in such a way that no messages are generated and thus no traffic is captured. Be aware that IP traffic is evaluated in order against the filter entries until an entry matches the IP parameters.
To check the IP Filters from the Enforce UI:
Global: Administration > Settings > Protocols
Server: Administration > Overview > <detection server>. Click Configure tab and select protocol in question.
You can validate whether the IP filters are causing an issue by removing them and then check to see if traffic is being reported through the monitor.
For example, a filter of +,10.67.0.0/16,*;-,*,* matches all IP traffic going to network 10.67.x.x but does not match any other traffic.