Are any of the syslog packets getting through? If some of the packets are coming in but you are missing some from randomly then you might be dropping them due to link congestion along the path from the router to the syslog server. Syslog packets are not automatically tagged so they would show up as default traffic in a QoS policy so are likely to be dropped.
If you are receiving nothing then I would do as bkyle suggested and run a packet capture on the syslog server. Another think to keep in mind is that the syslog server built into NPM can't handle a ton of syslog so depending on your overall syslog load that could also be an issue.