Not all sites displaying data in "Delivery Times Within Site/Administrative Group" report
This is a Server level report. Each site you wish to gather data from for this report requires more then one server in it to display data.
If data is desired for this report, there must be more then one server in the Site/Administrative Group in order for MessageStats to Report data. There is no way to tell how long the Message took to deliver within the Site/Administrative group if there is only one server as the message will need to leave the site to reach its destination.