Once you start flying a plane between city pairs, it can't teleport to fly somewhere else. You may have a plane flying Airport A to Airport B and have 100 available hours after that. If you go and try and fly that same plane Airport C to Airport D it will not show up because that plane never is in either of those cities. So while you may clearly have hours available the plane can't magically appear somewhere it doesn't fly to. If you add a flight that takes it from a city which it already operates out of like city A or B, and fly it even a single time to C or D you then will be able to operate C to D pending political restrictions.