Mailscanner/Sophos detecting fewer and fewer viruses every day???

[f ewf]

My mailscanner installation is exhibiting behavior which may be symptoms of
malfunctioning.  When I first installed mailscanner + sophos three weeks
ago mailscanner-mrtg reported 840 finds the first day, 840 the second, 620
the third, 420 the fourth, 250 the fifth, 210 the sixth.

First week instlalation numbers set aside, mailscanner has only found
around 60-80 a day during last 2 weeks.  I also have had reports of our
users receiving the mimail virus through the relay but it is possible that
the ide patch was delayed.  I have verified the existence of the Mimail's
IDE file for Sophos.  Additionally, when my user forwarded the message to
me the virus scanner picked it up.  However, with the wildness of this
virus I'd expect to receive more than 2 occurrences per day for 23k
messages relayed.

Are these normal virus detection patterns, or is something configured
incorrectly or malfunctioning?  What percentage of mail, on average, is a
virus?  Was there a spike of viruses net-wide the first week it was
installed (three weeks ago)?  Every time I send the eicar test virus is
picks it up.  Mailscanner-mrtg graphs are available upon request.

Thank you!