Why my girlfriend doesnt wanna have oral sex?

I've been knowing her for just about a whole year now and have been dating for 5 months she is the most committed girl I've been with I'm 17 n she is 19 when we have sexual intercourse she doesn't really like to go down on me I question her asking why she won't an she tells me she never has n doesn't wanna try it how can I change this also an why would she not want tooo?