To the person who has the partner who always quits, like it was already suggested; ask why and just stop if there isn't a good reason. Even I get to a point with people and go; "Look, this isn't working." One guy and I are great writers, and can make fun characters, but for some reason, and I explained this to him, how we design are characters...they just don't work together. It has nothing to do with an ability to write, but our character's don't flow. We tried 3 RPs, but I quit out every time because the character just didn't interest me and made the RP a bit dull. We parted ways as friends. It just happens sometimes.

But in her case, if she isn't saying anything, confront. Honestly, I'd say stop. It doesn't sound like she's able to commit to an RP.