There are settings that can control that on the Appearance tab in the instructor.
In this example, I specified a blank line setting with a "wdith" of 5 to make it very visible.
Here is a pict to help
I have the body set to a certain color, but in layout mode it's gray, in browse mode it's the color I selected
I have a solid line for the boundary, but in layout mode there is no line between the header and the body
My screen capture is in layout mode. Try increasing the width of the line. (I couldn't see any difference with a "line weight" of 1 either.)