Ian, there should be no problem with the moon being too bright. Think about it - your camera takes daytime photos; the moon is only just as bright. I think it has to do with your aperture setting. Point it directly at the moon and the camera should be able to auto-adjust the exposure.
Another reason why the camera may not come to focus is simply because of cloud, which would blur the features somewhat. Your camera will not be able to get a focus if that is so.
Cheers,
Chris