How can I treat patches of uneven glossiness that appear when using non-glossy finishes on wall surfaces?

This is known as 'flashing'. It's a fault in the paint which causes such patches to appear, especially at joints or laps. It is often caused by using paints ready for use rather than thinned or when the surface being paitned has uneven porosity. You'll need to repaint the surface with two thinned coats, first cleaning it thoroughly to remove all dirt, grease and surface contaminants. Then rub down with a suitable abrasive and dust off before painting.