1. She might have had something serious going on, like depression or a death in the family or something like that, and didn't feel comfortable telling us. When I was going through a really bad time in college, I never felt like I could tell my group members that those things were affecting my performance - I was afraid they would write it off as "just an excuse."
2. She might not have understood the project. When I was in college, I had several friends complain to me about their "stupid" classmates who didn't understand what was going on, so I would have never considered asking a group member for help when I didn't understand something. Since the other three of us knew what to do, she might have been afraid of saying the wrong thing.
3. She might be introverted or just not like to talk in groups. I'm willing to take the lead if no one else is saying anything, but if someone else takes the lead and talks a lot, I have no interest in competing. I hate competing to talk. I was once working in a large group in college and, even though I prepared ahead of time, said nothing at the group meeting because it was just too many people talking at once for me.
4. She may not have had internet access at home, which would have made it hard to contribute since we exchanged everything via email. We could have accommodated, but it can be very hard to say that you don't have something when everyone assumes that you do.
Perhaps some of my own group-work experiences in undergrad college - being the least interested, not understanding what to do, being afraid to say the wrong thing, not checking email 24/7, and just having a hard time keeping up - made me more empathetic towards people who appear to be free-riding. This was a temporary project, but if I ever have to work with someone like this long-term, I'm going to ask them if they need help.