Women Painters.com: Paint Issues and Fixes:

Poor Sheen Uniformity

Shiny spots or dull spots (also known as "flashing") on a painted surface; uneven gloss.

image courtesy of
The Rohm and Haas Paint Quality Institute

Possible Causes:

Uneven spread rate.
Failure to properly prime a porous surface, or surface with varying degrees of porosity.
Poor application resulting in lapping (see Lapping).

Solution

New substrates should be primed/sealed before applying the top coat to ensure a uniformly porous surface. Without the use of a primer or 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.