Keep on trying. It may take patience, practice, and some time before she's mentally and physically ready. She has to be real fucking turned on and so fucking into you that she really wants that dick inside her ass. You have to get her asshole opened up enough to take your dick, and it doesn't always want to open up a whole lot at one time. Open it up a little more each time you guys have sex. Then when the day comes that you have her little asshole opened up enough to get the head of your cock inside her, be very patient and very gentle. You're there. Once her fucking asshole is opened up enough for your dick's girth (thickness). It's open. Slide or slightly push it in very slowly and very gently, just a little at a time. When you get it all the way in, hold it there a minute before you start fucking her like you saw in the porno movies. Fuck her slowly and sensually, romantically, for a while. Then, and only then, can you bang her ass like she's a fucking bitch from the hood and you're the fucking porn star you always wanted to be. I've turned many women into ass fuck aficionados using this technique. Good luck to you.