I have been attempting to make a rough sketch of a gear to show someone just a little of what I know but I have encountered a situation where a line says that it coincides with the arc of a circle but it isn't connected and so does not count as a closed sketch.
You can see the coincident boxes where it should be attached but the rectangle is not shaded grey. Here is an up close view of the supposed coincident
Does anyone know why this is happening?
The answer given below is correct and fixed the issue
Welcome to the forum. I see you had the Power Trim tool open. If you click and drag across the portion of the circle between the vertical lines to delete that portion of the circle I suspect the whole area would become shaded, since you'd then have a single closed profile.
And in the second screenshot I suspect that's just a graphical error.