It's usually a humidity problem. Some recommend a gentle washing with a toothbrush to remove the excess matte material while others recommend a full removal and repaint. Matte finishes require little particles to break up the usual semi-gloss texture of the paint that is usually clear when properly cured. If gloss paint is like glass, then matte paint is like gravel, and too high humidity interferes with the curing process, which causes too much of the particles to collect on the surface.
When I first used clear matte paints in Hawaii, the finishes usually turned out bad, so I had to forego clear matte completely and rely on flat paints alone, usually enamels.