About 1/4 into a ride that we were all supposed to be together, I suddenly realised I was separated from Zwifters in my group and zwifting by myself. I disappeared from the leader board and will show as the only rider until the end of the ride. During the isolation my speed will sometimes go down to 6mph despite all my efforts. At the end, it recorded me as over 4 minutes behind. What did I do or not do? 
That’s the symptom of some sort of network connection issue. You lost connection to the Zwift servers. It’s your local PC/tablet/etc that’s drawing the scenery, and it needs data from the Zwift servers to show other riders and leaderboards.
Hi Steve
Ahhha…that makes sense. Very grateful for the prompt response. New to Zwift but loving my Zwift rides. Kind regards