Are you bending her over some surface, or do you have her on her knees?
If she's over a surface of some sort, try different surfaces (differing in height). If she's on herknees, experiment with the angle she's bending her knees, and how she's supporting her upper-body. I'm not going to claim to know what's better for different women, but I can tell you that changing those things definitely might help you-- I know from my experience that women react to the changes, and I can also feel a difference.
Have her mess around with propping herself on her elbows vs. lowering herself (though lowering herself will expose her to deeper penetration). Also... keeping her knees in place, move her forward or backward (to adjust how much of an angle she's making at her waist) and try from those.
One of my favorites with doggie is to get my gf resting on her shoulders, with her ass up, and then pull her back slightly, so her waist isn't at a 90... more like a 45-60 degree acute angle. Not only does it give a nice ass presentation, but it feels amazing. Only thing is it's kind've hard sometimes for the girls I've been with, especially if they aren't used to my size. My gf has had a lot of time to get used to me, but she still has days when she can't take me for very long.
I guess what I'm saying is that it seems sometimes girls are just a little more sensitive than other times. Just because you can't do it one day doesn't mean you might never be able to. Oh, and one final thing: make sure you're not going in too fast for her. Maybe it's hurting her because you're going in too fast and not giving her a chance to adjust before trying to fuck her.
Good luck bud. Doggie is a great thing ;-)