The percentage of women that like anal is quite low. So, maybe she's just naturally inclined not to like it.
You Might want to try to read up on it with her and see what the sexual benefits are and/or the pros & cons of anal sex.
However, if she's not the type, don't force it on her. Women that want anal sex will let you know. I've met them...and trust me, if they're horny, they will tell you, "fuck me up the ass"! And, they know what to do.
In regards to anal sex you can't just go in and hump her like a horse.
You really have to get her in the mood. She has to be extra horny and be the type that likes a good ass fuck. She also has to feel secure doing anal with you. She's giving up total control here...therefore, you've got to give that sense of comfort that at her will you will stop without any hesitation. Put yourself in her place and imagine what it would be like. TRUST is very important.
Please keep in mind that it is not a vagina and doesn't self lubricate, so, you need to use a lot of lube. Lots!
Start slow and take your time. Use toys or start off with one finger, work the muscle and communicate with her...again, she's got to be in the mood, relaxed and know that she can trust you.
If after all efforts she still doesn't want anal, do not force it on her.
If you feel anal sex is important to you, you may want to consider another partner.
Respect, trust and proper technique all go hand in hand, oh, and a woman that likes anal helps too.
I hope this helps and I wish you all the best. :hatsoff: