Outlook 2007 View by attcahment issue

[ghostcow]

Hi
I have a user using windows XP, Ms Outlook 2007 for several years;
For couple of days he's having a problem when trying to view the e-mails by attachment: "View>> Arrange by>> Attachment;
It seems like the arrange stops at a random e-mail , like:
/2011
........
/2009
........
/2008
/2011 again , al le-mails with attachments, different senders.



Please let me know what to do ..
Regards