From the various news sources it seems that the airlines have
assumed that the airports can handle their flight slots, but in reality the airports do not have sufficient ground handling or check in staff.
Flights are cancelled because the airlines are not ready to fly within the airline's take off slot window, so they have to cancel.
Of course the government is being blamed for not paying for
everlasting furlough for the airport staff during the pandemic.
Holiday industry should not sell what they cannot deliver.
