I can't find an attendee and I know they are at the event.
Attendee Lookup will only find attendees who have opted into the attendee directory by answering 'Yes' to the 'Show your profile in the attendee directory?' question in their User Profile.
If you can't find an attendee that you know is present it is most likely because they have opted out of the attendee directory.
Another possibility is that the search criteria your entered are not specific enough. A maximum of 25 names are returned for any search so you may need to make your search more specific to ensure that the attendee is included in the results.