Unable to sort by subject

I found a problem of sorting by subject in Outlook 2003. When I click a
message in the inbox, and then I clicked the subject button, Outlook should
all the messages by subject.

However, it won't work on some messages. Once I click the subject button
after I highlighted one of those messages, Outlook will still sort
everything by subject, but it just won't highlight the one I previously
selected.

When I am having a lot of email messages and I need to read them by
subjects, this will be very annoying. Is there any way I can solve this
problem?

Thanks......


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