Spamassassin score always 0

[Stephen Lee]

I'm runnig MS4.14-9/SA2.53/perl5.6.1/exim3.34 on a Trustix 1.5 system.
While I have "Use SpamAssassin = yes" and "Spam Checks = yes" set, the
Spamassassin score is always 0:

X-MailScanner-Information:  Please contact the ISP for more information
X-MailScanner:  Found to be clean
X-MailScanner-SpamCheck:  not spam, SpamAssassin (score=0, required 4)

Even though "Log Spam = yes", the maillog shows no evidence of
Spamassassin starting:

Apr 19 10:21:21 mail MailScanner[5544]: New Batch: Scanning 1 messages,
4645 bytes
Apr 19 10:21:21 mail MailScanner[5544]: Spam Checks: Starting
Apr 19 10:21:21 mail MailScanner[5544]: Virus and Content Scanning:
Starting
Apr 19 10:21:22 mail MailScanner[5544]: Uninfected: Delivered 1 message
Apr 19 10:25:01 mail MailScanner[5542]: New Batch: Scanning 1 messages,
2414 bytes

MS runs as user exim so I presume Spamassassin will write to
/home/exim/.spamassassin - even thought the SA whitelist is not called
within MailScanner.Conf. Could there be some permissions issues?

MS was installed from rpm and I followed the MS website instructions on
installing SA. What other diagnostics can I look for?

Thanks for any advice,
Stephen