Callbacks are often more about cultural fit than rigorous examination. I can only speak from experience in terms of what people were dinged for when I used to do firm callback interviews:
-Not being prepared. It's sort of a matter of calibrating yourself properly. Overdoing your prep can lead to awkwardness, but underdoing it or seeming disengaged means you're wasting everyone's time.
-Not having a good idea of where you would like to fit in at the firm. If the firm does lots of energy-industry work, don't play up your passion for Internet law. On the other side, it's all about calibration again. Your interviewers know you have no clue wtf actually working as a lawyer at any firm, especially theirs specifically, really entails, so don't feel too gunshy about that. Just give them the impression that you are a confident, engaged, and capable person and you'll have done everything you can.
Also don't forget interviewing is a fickle beast and numbers game sometimes. There were times when I absolutely loved a candidate and was pretty shocked at first when they didn't get a summer offer. Then with experience I saw just how common it was and how many places really do have the numbers where they have to turn down lots of good candidates. It happens a lot.
Turns out the woman I interviewed with had no idea why I was there outside of knowing the professor that recommended me. Applying for their internship program now, but I'm not sure if I can do it.