Job With UN

Why do I fail to get interview call for a job position with the UN despite possessing good experience?
I have applied for several vacancies in the UN through their site and have real rich experience (8 years) in fields of administration and management in the Indian Army as a CLASS 1 gazetted officer. Still none of my application has got me any interview call ! I am wondering why ! Are all or most of the vacancies filled with personal contacts in UN or the vacancies posted on the official UN site are genuine ?