How to make this work?
session times are not consistent, as the way you define your queue members you might have sessions or not at all. A static member never has a session start or stop, for example. Others think that starting or launching a softphone is 'enough' to start a session (Which is not true).
So, if you want to change what is done, you can adapt the getstatsfromasternic.php script included in the full wallboard plugin. It is commented, it peforms mysql queries to get data and try to determine session times. IT will read for add/remove queue member events and use that as a base, and it will try to fill the gaps if those events do not exists.
So, there is no answer like "do this to make it work", because it is heavily dependant on how you use and configure your system, a thing that *you* solely know.