The magnetic tracker will not always snap to the center. When you hover over the arc, you should see a crosshair cusor with the "center snap" icon attached, as in the screen capture I've attached to this message. You should also see a small red diamond at the center of the arc. When you do see this display, click the left mouse button to establish the center of the circle you wish to draw, then drag the cursor to start generating the circle. Your next mouse click (or your entry in the Inspector Bar) will establish the radius of the circle and finish its creation.