Hi,
Do you have AutoArchive set to run automatically?
I.
(1) Select 'Tools->Options' menu choice.
(2) Click 'Other' tab.
(3) Click 'AutoArchive' button.
(4) Is 'Run AutoArchive every' checked? If not, check it and choose
the number of days.
(5) You can also check 'Prompt before AutoArchive runs' to have
Outlook prompt you before it attempts to archive older items.
You can also attempt to run archive manually.
II.
(1) Navigate to 'Sent items' folder.
(2) Select 'File->Archive' menu choice.
(3) The 'Archive this folder and all subfolders' choice should be
selected by default. The current target should also be the 'Sent
items' folder. If not, select it from the list.
(4) Or if you prefer, select the first choice 'Archive all folders
according to their AutoArchive settings'. This will save you the
trouble of specifying a date for the 'Archive items older than' field.
Note: This may be a problem if you've specified AutoArchive settings
on other folders, but do not wish to archive items in those folders.
If that is the case, you'll need to disable autoarchive for those
folders.
(5) You can also check 'Include items with Do Not AutoArchive check'
if you like.
(6) Click 'Ok' button.
Cheers,
V