Maybe add one to two less weekly frequencies. Sometimes there's a glitch in the game say "aircraft #### needs 140 hours has 139.999999" or something like that. I've encountered many of these errors and to fix it just put one to two less frequencies a week. Not too many.
It's worked on some routes. However the glitch doesn't allow me to add any flights to many key domestic routes. For instance, I can't even add a single weekly flight on PEK-CAN, which is a route that should be earning me well over $2m a day.