How many idle aircraft do you have? How many total aircraft?
I find that in the late game when my fleet is quite large, and I have lots of idle aircraft, I cannot add flights. In fact, I cannot sell aircraft or even view the used market. I have noted this issue in the bug tracker, but with no reply from the developers. It is almost certainly a server resources issue.
I would be happy to pay for better server space if there was some way to do so. I want this game to stay alive, and the quality does seem to be slowly decreasing.