Facetime starts, says "connecting" then times out. Both sending and receiving. ?

I have used Facetime in the past successfully with people with whom it will now no longer work. Both when I initiate the call and when they do, we get a connecting message at each end, lasts a long time, and finally times out with a "not available for Facetime" message when it stops trying. It continues to work for these people when the are contacting others, but not for me with them. I have an internet connection, other net related things are working. Seems like this began when I upgraded to El Capitan, OS 11, but I did not try FT immediately after the istallation.