Poor sheen uniformity is generally a result of inconsistent gloss levels and will appear as shiny spots or dull spots on a painted surface. It is also known as 'flashing'.
Uneven spread rate
Failure to properly prime a porous surface, or surface with varying degrees of porosity
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, an extra 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.