It's very easy to say it's training issue or individual incompetence.
That is very rarely the case isolated. If you put the individual, well rested, in the frame of mind that they are in a training environment, they will perform to standard.
But the real environment is different, crew are operating longer days, with more disruptions, with more lifestyle changes, often at anti social hours, couple that with passenger issues, hand baggage issues, busy flights, corporate pressures e.g. punctuality; then you have a whole myriad of factors which could affect the individual's performance at that very moment.