In outlook it is possible to organise your emails in your inbox by filtering them.

For instance you can tell it only to display emails that are unread or that have been received in the last seven days.

If you have many emails missing in your inbox it may be that you have inadvertently applied one of these filters.  To show ALL of them again go to pull-down in the advanced toolbar and select Messages (first in the list)

If you can’t see the pull-down – RIGHT click over the toolbars and select Advanced with a left click.