Most women would enjoy anal sex ( easy for me to say!
), but there are many factors that create fear that's hard to overcome.
#1 is it will be too painful, followed by only "nasty" girls do that, and it will 'damage' their asshole. Then you have to create a scenario where that could happen, at the right time, at the right place. If you can overcome all that and actually attempt/achieve penetration, your technique has to be pretty much perfect, not too hard (at first, anyway) and not too fast.
But strangely enough, once they try it, they can really get into it and get off on it.
As my wife aged, she decided she didn't want cocks or dildos in her ass anymore, but still loved taking a finger to the knuckle.
She had a NICE ass, and guys she fucked would get off on it enough to want to fuck her ass. And, as a general rule, she would take it from them as long as they weren't rough. After we met and began dating, the same was true for me, also. So I'd fuck her in the ass regularly; at least once and sometimes twice in a week, then eventually slowing to about once a week.
But some 20 or so years later, she decides that she doesn't want her asshole to be "ratted out" (her words) so don't put it in her ass anymore. Still loved the oral and manual stimulation, and would take a finger as mentioned above. Some times, I would rub her asshole with my fingertip, but not apply any pressure, and she would still hump and push against it enough to make it slip in.
Anyway, keep trying, and best of luck.
Tell her I said it'll be OK.