girlfreind won't give head

I am 26 years old and my girlfriend is 34. We've been dating for a little over a year now and I've asked her a few times if she would go down on me. Each time she refused.  I could understand if she doesn't like the taste or had a bad episode once but thats not the case.  She says shes never tried it. But thats all i could get out of her. Every time i ask her why she hasen't tried it or what bothers her about it she competely shuts down.  Shes done this since the very first time i asked her.  Won't speek a word about it. I have no idea what to think.