I would say that your best bet is just to ask her how she feels about it. If she's hesitant, what is the reason? Some women just don't like it, or have small mouths or strong gag reflexes that make it unenjoyable for them. She may do it because she wants to please you but maybe gets no enjoyment from it herself. Do you discuss your sexual desires openly in your relationship? If so, bring it up by saying, 'is there anything i can do to please you?' and let her know what you really love and what she can do. I think if you can get to the bottom of her feelings surrounding it then you will understand where she is comng from. Good luck!