Sometimes we have to spend about an hour talking her into going along.
Stop feeding her. If her character does not want to go along? Well, then the character does not. That's her decision. There is no reason the others should not have fun. Her character does not want to investigate? Well, then the others do it alone and have fun.
Do not argue with her. Simply show her the consequences of her actions. The consequence is nothing bad for her character, but she is missing out on the game. She is not part of it.
