I’ve recently encountered a problem, where I’ve leased Gates in TXL, MUC and FRA and created TXL-FRA/FRA-TXL and TXL-MUC/MUC-TXL routes but my aircraft only flies the TXL-MUC/MUC-TXL legs. For the other routes, it just remains stationary at TXL. The schedule fits and adds up, so that can’t be the issue.
Have you tried combining your routes into a single “route” instead of individual pairs?
With the way scheduling works in the game, you should think of each route as a complete day schedule for a single aircraft.
You should have all flights in a single route, like this:
A-B-A-C-A, which in your case is TXL-FRA-TXL-MUC-TXL
Having two separate routes, assigned to the same aircraft, only works if the routes fly on different days.