Shiny spots or dull spots (also known as flashing) on a painted surface; uneven gloss.
New substrates should be primed/sealed before applying the topcoat to ensure a uniformly porous surface. Without the use of a primer or a sealer, a second coat of paint will more likely be needed. Make sure to apply paint from 'wet to dry' to prevent lapping. Often, applying an additional coat will even out sheen irregularities.
Source: The Paint Quality Institute