Poor maintenance service Philwest

My partners car was booked in at Volkswagen Philwest as the car struggled to start and the EPC light appeared. Car was returned the 30th of Dec. 31st when we had to leave for our holiday the car gave exactly the same issues after 4 spark plugs, 1 fuel injector and a fuel filter was replaced. Car was brought back to Volkswagen and now the feedback is the other fuel injectors will have to be checked and possibly replaced. How is that possible as all necessary checks had to be done when the car was initially brought in and that other fuel injectors can break within less than 24 hours!