Sent e-mails are not showing the date

I am using Outlook for Mac 2011 Version 14.2.2. In the list of sent e-mails older mails have a date and time stamp, but for a period of about 2 weeks sent e-mails do not show the time stamp anymore. They are listed underneath all other e-mails, when I sort by date. Also the search-function is not working properly anymore. When I search for a sender of an e-mail, it does not list any e-mails at all (even though I have received hundreds of mails of this person), next time I perform the same search, it finds part of the mails.