My co-captain isn't helping-what should I do?

Maybe set a date when you can work on it together, eg. 'Hey, when are you free this week? I think maybe we should work on that fundraiser, cuz it's always more fun doing it together.' Or you could organise one time a week where you can get together and do it. That way you can get her involved but without having to confront her about it. Hope it helps!