This is not a problem that we hear about often, so it appears to be an infrequent problem. We are not currently aware of any situations where any users have ongoing/permanent problems joining sessions. It appears this happens when the JamKazam application somehow gets itself into an unstable state.
So if you find that you cannot join a session, we first suggest that you close the JamKazam application, then restart the app, and attempt to join a session again.
If this does not fix the problem, then we suggest that you reboot your computer, reboot your home router, and then restart the JamKazam application and attempt to join a session again.