A lot depends on prepping the substrate; you need to sand the wood smooth, apply a sanding sealer or other sandable primer, sand that smooth, and then apply the paint. A thick nap roller will leave a bumpy finish, where a thin nap will lay on a more even coat.

Wjca is right about spraying being superior, but I will assume you are not set up for that. You need a commercial type sprayer, not from a can.

At this point I would wet sand with 220 grit to get a smooth surface. Depending on what kind of paint you used, and how long it has dried, the sandpaper may gum up if you don't wet sand with water. Once you get the surface flat and smooth again, recoat with a thin nap roller in a dust free environment.

If the paint is a semi gloss or satin, you can always dull it down with ultra fine grit wet sanding. You need special paints or other surface coatings to be able to increase the gloss by rubbing out and polishing.

Putting polyurethane on over the paint can increase the gloss, but oil based polys will change the color somewhat as they tend to "yellow" it. Water based polys tend to change the color less.