By default the desktop client version of Outlook will only show you emails in your mailbox up to 1 year old.
In Outlook click File > Account Settings > Account Settings to configure options for your mailbox
Select your mailbox and click Change
Move the date slider to the desired age range and click Next
On the next window, click Done
Close Outlook and reopen it for the change to take effect. It may take some time to sync the changes and if you have access to a shared mailbox this may take 24 hours to sync.