Filter rule only works part of the time

My ISP filters my email and adds ***SPAM*** to the
beginning of the Subject line on spam messages. My
filter in Outlook 2002 is set up to read ***SPAM*** in
the Subject line. The filter works sometimes but
sometimes is does not work. When it doesn't work, I can
run the filter mannually and then it will delete the
***SPAM*** messages as the rule is written.

Does anyone know why the filter only works part of the
time? Thanks!