Set your PIX logging level to Debugging (Level 7):
pix(config)# logging trap debugging
I guess you have built a conduit (or ACL if you've moved on from conduits!) for this connection. If you have configured the PIX as per the manual (it says for production use level 3 / errors) then the syslog will only contain denys etc. If your firewalls are busy, then the logfile will become very large once you set the syslog level to debugging.
Anyway hope this sorts it.
Phil.