[exim] OT - Not all messages being checked for spam

I've got a problem. A lot of spammish emails are getting through, and
looking at their headers reveals that some of them have X-Spam status of
"Not checked" and others don't have X-Spam headers at all. How can I
ensure that all mail is checked for spam? Here's my ACL:

acl_check_data:

  # Deny if the message contains a virus. Before enabling this check,
you
  # must install a virus scanner and set the av_scanner option above.
  #
#  deny    malware    = *
#          message    = This message contains a virus ($malware_name).

# Add headers to a message if it is judged to be spam. Before enabling
this,
# you must install SpamAssassin. You may also need to set the
spamd_address
# option above.
#

warn message = Subject: [*SPAM*] $h_Subject
spam = nobody

add_header = X-Spam_score: $spam_score\n\
X-Spam_score_int: $spam_score_int\n\
X-Spam_bar: $spam_bar\n\
X-Spam_report: $spam_report

# Accept the message.

accept