I was abducted and raped just over a year ago, and I didn't report it, either.
I was 18 at the time, and I'd only just moved back in with my father after being estranged from him for six years. I didn't want him to find out, and I didn't want to have to re-live and re-tell every sick, disgusting detail to some police-officer I didn't know, or be examined by a doctor. I never told my counselor about it because I couldn't, and still can't, bring myself to discuss it out loud.
I don't think feeling like this is uncommon. You shouldn't attempt to push her into it, reporting rape is like having to go through it again. Perhaps she doesn't want to have to describe what happened, or doesn't want it getting out, or other people in college knowing. It's her decision, and you need to respect that. I'm sure she won't thank you for reminding her about it if you keep asking her to take action.