If you use an iPhone and appointments created on the phone are not visible in Outlook’s Day/Week/Month view but do show in the By Category (or any table) view, try this:
Open the event on the iPhone and change it to recurring daily then end the recurrence on the date of the event before syncing.
Let me know if it works for you.
More information is available at iPhone, Outlook, and Disappearing Appointments (slipstick.com)
Published July 18, 2008. Last updated on March 27, 2012.