How to deal with an unreasonable roommate?

I’ve just started my junior year of college and after two years of bad roommate situations, I thought this year would be the jackpot because I was living with two of my best friends. My two roommates went to high school together, and one of them has a bone to pick with the other. Roommate A doesn’t like it when Roommate B leaves on the weekends to visit her long distance boyfriend; she wishes that she would stay and hang out with us (but she only visits him once a month?). Because of this, roommate A comes home and completely ignores us and is passive aggressive to both of us (even though I’ve done nothing but be nice to both of them). B has already confronted A and we thought everything was solved, but we’re right back at square one. I’m frustrated because they’re also my friends and I’m stuck in the middle, and I feel like A has no reason to be mad at B. I’m very stressed because I don’t want to have ANOTHER miserable school year. What can I do to fix it, or should I just leave it alone?