Google Analytics sometimes show 0 Active users and 0 page views, despite the rest of the Google Analytics data showing otherwise.

This is because of the dreaded "index.php" tracking issue.

If you check Popular Pages on the dash tab all the URLs have "index.php" at the end but if you go to an article on their site it terminates with a / and not index.php

In your Google Analytics go to Admin, then View Settings (under View, 3rd column) on that page there is Default page (optional). Most likely you have "index.php" in there. If you drop it, things should start acting normal