What Causes Pain In Vagina During Intercourse In Spite Proper Lubrication?

My girlfriend experiences pain and lack of excitement during intercourse. We've had intercourse about 20 times, and symptoms have been present every time. We've both done our research, but nothing seems to fit the causes. She does get aroused and properly lubricated, but she still experiences substantial discomfort. What could it be? I there anything we can do to isolate or fix the cause?