OK, I find that if I display the atoms in spacefill at 50% the monitor lines appear. The problem with the monitors not appearing was a monitor between two alpha carbons that were in a cartoon display. I can get around this by displaying the atoms at a small spacefill size, but it would be nice if monitors showed up when alpha carbons are measured in cartoon displays. Particularly when using the mouse, it's kind of unnerving to have the line be there when measuring and then simply disappear when clicking to finish. Would it be simple to change this behavior?