It sounds to me like when she approaches the orgasm, she somehow blocks her in her mind.
Either she's thinking too hard about achieving an orgasm with you, which will just push the orgasm further away, or when she's approaching the orgasm she's withholding it for some reason thinking "oh I dont want to come yet" & it spoils the moment & sets her back to step 1.
The psyche is a powerful thing when it comes to an orgasm, I know the few times I don't get there myself, its purely because I block it in my mind.
I think you should talk to her...ask her about past relationships & orgasms... Was she ever with a guy who thought she took too long to orgasm? Does she feel that YOU think she takes too long to reach an orgasm & she's trying to rush it? Did she have boyfriends who were really quick & never gave her time to orgasm so she's trying to reach it before you are done?
I think the most important thing is to distract her from what is going on .... put on some good mood music, put a porn movie on the TV and get her hooked while you caress her, talk to her while you caress her, take her mind off of what is going on in the nether regions while you build her up...
Also might I suggest teasing her more.... bring her to a point where she is about to go over the edge (that step where you feel you have to go back to step one) then withdraw BEFORE you get to that point...go back & tease her again after she calms down, if you do this a few times it'll be almost impossible not to go over the edge, she'll be frustrated and very turned on.
I think once you achieve an orgasm, it'll blow away whatever mental block she has about orgasming...
and it will be easier if she's able to orgasm a few times, then it won't be on her mind so much anymore & it won't be so forced.
So talk, distraction & teasing ... Good luck, I hope she can get there, I know how frustrating it can be wanting to orgasm & not getting there.
Tig
PS: Also make sure the room is cool. A lot of times if it gets too hot, it really gets hard to orgasm even if in the mood