Can flat paint be painted over latex satin wall paint?

Yes, you can generally paint high gloss latex over flat latex paint. But you must do all the preparation work in the best tradition of any professional painter. All "flaking" must be removed and the raw wood primed. This will present a "smooth" surface for the new coating. Additionally, everything that is to be painted my be scrupulously clean and free of anything that will restrict the new latex from sticking to the old.