In collaborative fiction writing, as in life, not everyone is going to be the right 'mix' for your project. That doesn't mean that they're not a good writer or that they are a bad person. It just means that their personal style or their vision of the story is not working with the rest of the team. There are some simple things to do to try to get everyone working on the same page.
First of all: listen. It's essential to give everyone a chance to express their ideas and creative vision. If you're one of the project leaders (we call them Members of the Board at Pan) and this was your idea for a story you might be tempted to come down hard and insist that it's your way or the highway, but if you do you're also likely to be writing your story all alone. Sometimes even the most difficult writer will come up with ideas that improve on the original concept. If you're not open and you don't listen it will never happen. The case, much of the time, is that people are way off base. Say you want to write a hard core survivalist story and they start coming up with some more fantastic ideas like mega-warriors with super-powers that are hyped up versions of Mad Max on steroids. Hear what they have to say, see if there are any parts of it you can use, and then be firm but supportive. The fact is that what got them excited about your story might not be exactly what you had in mind.
The goal is to bring them closer to the concept without stifling their creativity. So in a realistic survivalist story a band of Mad Max types might fit, but tempered down to earth - would that work? Consider the idea before you just out and out dismiss it. With some working it might fit in - or not. But be sure you have listened first and not just playacted at listening. Keep a respectful attitude and a gentle demeanor in your written communications. In writing people can't see your facial expressions or hand gestures. Your respect needs to be OBVIOUS from your word choices and sentence structure. Unfortunate word choices can alienate. Always reread your communications before hitting the send button and NEVER respond in the heat of the moment if things are getting a little hot under the collar.
It's the case in all collaborations where someone with a very differing view of the story is going to decide to walk. That's ok; it happens. When it does you still have to be respectful, and it's ok to let them go. As long as you've done your job of listening, trying to work with their ideas, etc., as a team leader you hopefully avoided any negative conflict and commentary that can spoil the fun of a good collaborative role play writing project. This story might not be the right one for them but, who knows, another time you might find it fun to work together. Try not to burn bridges!
All the above advice can apply to the writer that is having trouble fitting into a story that initially interested them. You might need to be flexible if the story is not exactly what you imagined when you applied to join in, but remember it's ok if not every story is a good fit for you. Just approach every new story as a potential team member. It's not just the vision that you see in your head of the character. It's the sum of the parts, not just the individual parts.