Credit to https://unix.stackexchange.com/a/12772/61349 for their diagnostic instructions.
Ensure you're ssh client is requesting X11 Forwarding with the -v flag
ssh example.com -X -v
And look for thespecific message:
debug1: Requesting X11 forwarding with authentication spoofing.
Another telltail problem sign is the lack of the
DISPLAY environment variable beign set for you. If its blank/unset, there is something wrong. At least according to the same post credited above, these variables are set for you automatically.
I was using an SSH Control Master my ssh connections like so:
My earlier ssh connection did not request X11Firwarding, therefore future
ssh call's options weren't being used. So for me,
-X was just being ignored altogether
If you aren't seeing "Requesting X11 Forwarding", but you do see the muxer reusing sessions;
debug1: auto-mux: Trying existing master
debug1: mux_client_request_session: master session id: 10
Then you need to exit your current ControlMaster for that connection and reconnect with the -X/-Y flag.
To Stop your ControlMaster
ssh -O check <hostname>
Master running (pid=2758)
(This will exit all active connections using the ControlMaster)
ssh -O exit <hostname>
Exit request sent.