Network Rail Customer Service Assistant interview questions

Applied end of July 2017 this was via an online application, followed by an online multiple choice questionnaire. It was the beginning of August that I got communication to say that I was successful and that I had been invited to an assessment centre end of August 2017. The date that they had given me was not changeable and had to be done on that date as there was no over alternative dates. 1 day after the assessment centre was told I was successful and was invited to a final round interview which consisted of a panel with the Customer Service Manager for the end of September 2017 again there was no alternative dates for the interview apart from the one stated. A week later was told I was unsuccessful. However was confused as to why I didn't get the job as I have worked in customer service for many years. So I was fully competent with the questions they asked me. Also, the job had stated for 6 customer service assistants and when I went to the assessment centre only 5 people attended which included myself in that group of five. In which I was a bit baffled as to why I didn't get the job. I did call their HR department to ask for feedback. However, there was none left by the customer service manager. Just to say I was unsuccessful. I did ask if she could contact the hiring manager as to why I was unsuccessful in which she replied it's down to the manager's discretion.