Little things can make a big difference to how problems are perceived...
.. the passengers were told that the plane had finally been repaired and would be leaving at 8 p.m. But after departure time came and went, the passengers were then informed the flight had once again — this time due to flight crew issues
To passengers that appears incompetent - from their perspective the airline knows when the crew will be out of time so why tell them to wait around for an 8 pm departure if their won't be a crew?
It sounds like the airline knew the crew would be out of hours if it didn't depart by 8pm and just hoped the engineers could fix it by then. If you actually tell passengers what your plan is and that it might not work out then they will/should be more understanding when you don't quite pull it off..