I found the solution to my problem (live weather option greyed out on in flight tool bar - even on new flights) on the avsim forum.
For me this issue started when I tried Multiplayer (which I don’t usually do) and then turned multiplayer off again in the main (data) menu, as is my normal preference. Afterwards, live weather wasn’t working for me.
I am sharing it here in case it helps someone else. Apologies in advance if this has been posted previously.
Solution below:
"Had the same thing yesterday after I turned multiplayer on and flew around Detroit. Then I turned it off, and had no weather icon. After trying a lot of things, I did the following this morning, and I have the weather icon back.
1).Turned ON Live Multiplayer and Live Weather.
-
Set up an airport to depart from. On that screen, I clicked on ALL Players for multiplayer. Weather was set to live. Clicked on the Fly Button
-
Once at the airfield, clicked on FLY. Now I had the weather icon back. Opened it up, and set the weather to Clear Skies. Hit ESC and clicked on Restart at bottom menu. When at airfield again, weather icon was still there.
-
Exited flight, and clicked on General Options, Data, and turned Live Multiplayer OFF. Left Live Weather ON.
5). Weather icon is now working again."
Following the above steps fixed this issue for me.
Cheers.