Check the persons ticket. They could of brought the wrong ticket. This will be indicated on their ticket and in their confirmation email.
Only one event can be downloaded at any point in time.
From the scan tab, you can see the name of the event currently downloaded to the phone in the bottom left corner.
Confirm this is the event you wish to be scanning. If it is not, tap on "Close" in the top right of the screen, login and select the correct event.
Confirm the list of of ticket types that are filtered by following the filtered tickets guide. If you didn't want to filter the scanning disable filtering by navigating to the Settings tab, and set "Apply filter to scans" to OFF