A curious thing about satin/matte paints: they are actually exactly the same as gloss paints, only with a matting agent added. This matting agent acts like a bunch of tiny little lenses that cause the light to diffuse about in random directions as opposed to the light bouncing right off like on a mirror. Growing up, I always assumed it was the other way around, that gloss paint had special shiny chemicals in it that would rise to the top of the paint job.
Anyway, the point is, when working with matte paints, it is extra important to shake up the paint really really well, and continue shaking the can between every few seconds of painting.
Otherwise, yeah, as said, easiest to increase the matte by either scuffing with something high grit. A scouring pad, mr. clean magic eraser, 1000-2000 grit sandpaper + water are all fun to play with. Or, you can paint it with a clear matte topcoat.