I'm not 100% sure, but I think your main problem lies with poor character development. You need to not only really develop each character's back story and personality, but make sure it ties in with the rest of the party AND the main quest.
For example, I created a pair of characters with a friend for a DnD game when I was younger, he was half-angel and I was half-demon, we were like two halves of the same person and couldn't wander more than x miles from each others because of magic. Sounded interesting on paper, but it got old very fast: we were really just splitting the party with infighting and the quest went nowhere. So your characters need not only be interesting by themselves, but bring something interesting to the party. What are their values, opinions, what do they/will they think of other members? How will they interact? Will it actually help the story go forward and be more interesting or just detract from it?
About my second point, characters need to have a reason to join the party. Your party is likely on some sort of epic quest. Maybe the hero or some other dude does it "to save the world", but most characters will do it for something else entirely and maybe save the the world at the same time, as an option. Unless it's a joke game or the story is very light and unimportant, no character will likely join the party for the hero's nice smile or just for kicks. Or at least not in the long run. They will all have more or less assumed motives that more or less ties in with the plot. About your character that's been left at the altar by his pregnant wife or something, what does that have to do with the quest? Will the quest bring him by a town he knows his almost-wife to be? Or will it allow him to flee and hide from her, if that's his thing? They can also have multiple motivations, some they don't even know of yet: say that character fell in love again, maybe with a party member, and wants to help her out, and along the way he stumbles upon his ex-almost-wife. I don't know, anyway, whatever.
So yeah, I really gotta go, but to resume: plan your story ahead, at least in broad strokes, and do more character development. If the hero or any other character is hogging the spotlight, maybe it's because the others are not spotlight-worthy. That or poor writing, I guess...