You need to verify that syslog is configured to forward the traffic logs.
You can verify this by checking it via the "get syslog" command. Make sure that the traffic log is enabled.
If not you can set this up by either defining it specifically or by enabling all logs.
set syslog config x.x.x.x log traffic ( you can also make this more expansive by using "all" instead of the keyword traffic )
set syslog enable
save