E.g. your first image and fourth image dont have specific backgrounds. You don't convey any depth: There is no distinguishable foreground and background, just your characters. Ask questions like: Where is your character, in what situation, how does the environment look like? If you need inspiration, go outside, take some photos. You can even use them for reference. The artist of Scurry gave some good advices on background on his Facebook-Page
For these I collected some nice tutorials on my Patreon. They explain how light affects objects. For example you can use indirect light for outdoor settings. Back light is also really fun to make. Important tipps: the sky doesn't need to be blue, the sunlight isn't white, and grey is wonderful to emphasize color.
For example I recently made this picture:
As you can see my shadows aren't black but grey. It supports the colors that are illuminated perfectly. The light is mostly yellow-ish to get a warm feeling. The background is cold to contrast my character. Care that the further away an object is to the viewer, the bluer it gets. Use that to convey depth.
I hope this helps