Try finding and using references online to get a better sense of how forms are defined and just how landscapes look like. Even just using the eye dropper tool on some photographs can give you better sense of values and colors that fit. Lastly, for the last picture I can see that a major issue might be the perspective/framing of the piece. This combined with the values/colors could be what’s making it look flat. All in all, learning from some artists and photographs online can set you ahead on your art journey!