Why don't Opaques mask objects behind them on some drawings?
Opaques will only mask the objects behind them if the display mode is set to 2D Wireframe. This is controlled by the icon at the bottom of the screen. Click the icon to change the setting.
Objects on planes above the Opaque (typically drawn after it) will not be masked, unless you change the display order to move the Opaque plane above the objects (or move the objects plane below the Opaque). You can change the display order using tools in the 'Edit' Menu