Assuming they are simple lines and a circle, you need to magic wand
inside
the area, not on the circle itself.
This goes for any magic wanding. If you magic wand on an outline, it will trace the entire outline. If you magic wand inside, it will find the nearest valid boundary, assuming there is one.
There are two different shaped magic wand shapes to show the difference in methods - one kinda rounded and one with sharper edges. Try it out and see!
Cheers,
Link.