Do you need a reference point for where your light is coming from?
Light and Shadow
Change your piece to greyscale, and squint your eyes. Are the values of each area distinct, or are areas blending together with others?
Do you need to set the piece to greyscale to focus on lighting and shadows? Make your piece greyscale. In Photoshop: View > Proof Setup > Custom… Choose “Working Gray – Dot Gain 20%” Press Ctrl + Y on Windows, Cmd + Y on macOS to toggle the greyscale. Try sampling colors and adjusting them …
Do you need to get more lighting reference?
Do you need to mask in lighting? Create a layer filled with light color, set to overlay, and mask in lighting with a soft brush.
Do you need to use screen/multiply clipping layers to paint in shadow and light?
Are you paying attention to your light source? Is every element lit in the same way, or are you falling into bad lighting habits?
Are your objects getting too bright for their local color? Light makes an object as bright as it can be, but no brighter. An orange pumpkin doesn’t become white in light—it can’t get any brighter than 100% orange.
Are you lighting from the top-right hand corner? Did you mean to, or are you just being lazy?