Autofocus needs contrast to work. Low light venues are typically lacking in contrast, unless there happens to be a light source visible in the area autofocus is seeking, and even if there IS that may not be where you want focus to be. I always use MF in low light; ALWAYS. I don't trust AF in low light.
Some flashes/cameras can send out an infrared light that will measure contrast and help to auto focus. I know some Sony cameras have this. There's a add-on to extend the range available for them too.
