After a PST is successfully migrated into Exchange or Office365 a User might see the correct PST folder structure in their mailbox, but not all the migrated email items will be displayed in Outlook.
This behavior can be caused by the Offline Settings in the Outlook client when using Cached Exchange Mode. This setting (File>Account Settings>Email>Change) controls how much data is synchronized with Outlook, and is based on the age of the mail item. In the screenshot below, the ‘Mail to keep offline’ slider is set to 12 months. This means that anything more than a year old will be kept on the server and not synchronized with Outlook, resulting in apparently missing emails.
A more detailed description of settings and behavior can be found here.