It’s a two-fold problem. One has to do with knowing what you’re trying to communicate as an artist, and the other is whether the person even has enough artistic knowledge/skill to execute an effective thumbnail sketch.
One of the most common problem I see in students’ work, is that many have no idea what they even want to communicate as a creative talent. They just sort of create without really knowing why they’re doing it. They don’t think about what moves them emotionally, what mystifies, what angers them, what frightens them, what memories haunts them or brings them joy, or have anything to express about the world we live in. Of course, that’s very personal stuff, but even on just the level of entertainment and escapism, they don’t think about visual narratives and the mood they want to convey. What is the visual story they’re trying to tell? What is this fictional premise all about? What is the emotion being expressed in this fictional scene? What is the “selling point” of this image’s visual story/mood?
The other problem, is the lack of artistic knowledge/skill to execute effective thumbnail sketches. The person has no adequate understanding of perspective and composition, so he can’t even sketch out the scene using interesting camera angle and field-of-view, with optimal camera placement. There’s also no effective focal point or spatial arrangement in the composition, and there’s no sense of visual flow or hierarchy of visual weight. He has no grasp of value management and effective lighting, so his sketches lack any kind of planning in lighting and values and readability of forms. He has little to no understanding of color theory, so there’s no cohesive color palette or effective portrayal of mood using deliberate color schemes. And the lack of knowledge in anatomy/figure, expressions/body language prevents him from sketching out natural and expressive looking characters/figures in the thumbnail, even if they are relatively quite small and don’t require much detail at all.
So in order to create effective thumbnail sketches to plan out the image, the person would need to address both of the above issues.