Hi,
I’ve experienced a similar issue, however, I noticed a very quick error message in the upper right corner that indicated the failure to save the file.
If that’s your case, I have the fix for that.
The cause is simply you’ve exceeded the save limitation, and must start deleting the online flight plans you’ve been saving in order to get them into the game. So you’ll be able to save again once start deleting some flights plans to make room for new ones.
I didn’t count, but there are quite a few saves available until you hit the limit, could be close to 75-100 saves.
I suggested to support that it would be nice to have the ability to save the flight plans locally, or our MS Cloud Shares.
Good luck! Hope that helps 
Let me know.
Thanks!