When easyJet file the flight plan with CFMU in Brussels they'll check it before they acknowledge it. If there is a mistake (the aircraft is planned to fly down an airway which is closed etc) then the central flow management unit will reject the flight plan. Sometimes it's quite hard to find an alternative route which could have caused the dealy. It's been known for CFMU to close the wrong airway which can cause mania when trying to find alternative routes.