A while ago, I started writing a short story for a competition. It was supposed to be about four girls in a shared student apartment. The plan was to have everyone conflict with everyone until they united against a common enemy (the landlord or the exams - I haven't reached that far), and learnt to put aside their differences.
I soon ran into a problem: one of the girls, a vampire, had much more oomph than the others. She was in conflict with the werewolf girl because werewolf, she was in conflict with the Ifrit girl because the latter belonged to the nationality whom the former blamed for being dead, she was in conflict with the religious vanilla POV girl because a religious issue is how she ended up a vampire instead of being dead-dead. Most of all, she was in conflict with herself, because dying and waking up a vampire really threw a wrench into the plans she had for her life. She was a walking-talking explosion.
The other girls had their conflicts, but those conflicts were mundane. Vampire - she was boiling over, which caused her to dominate every scene, and every scene without her became drab by comparison.
I tried to amplify the others' existential anger, but then there was so much conflict that the story got mired down in arguments, and wouldn't progress. I tried to make it Vampire's story and get rid of the others, but that didn't work either - she needed the other girls to bounce off, they allowed her to let out some pent-up rage. They "represent" in a way the people/bodies/situation she's angry with.
How can I address such a situation, when one character in a group is drastically overshadowing the others? Please note I am interested in general approach answers, in the spirit of the two examples I presented in the previous paragraph. I am not looking for specific "write this" answers - those are off topic, and wouldn't help anybody else, nor me when I run into such a character again.