The “literary” versus “commercial” distinction runs deeper than vocab and sentence structure, so elevating the language won’t address the agent’s concern. I suspect your concept promises rich exploration of themes or sociocultural issues, while the story itself is action- and dialogue-driven, having the effect of skimming the surface of those themes or issues. I hear the agent calling for richer layering, with more nuanced character work as you explore how sociocultural elements of the era affect your character and, thus, her interactions with others. Does your protagonist act and react to others in discomfiting ways that force everyone to question or defend worldviews beyond the event at hand? Consider To Kill a Mockingbird, in which a child’s fear of the bogeyman plays out against the larger canvas of a town’s railroading of a black man. As the characters confront the overt theme of racism, they also struggle with universal themes of courage, class, gender, and compassion. Layers. Literary. Above all, rich storytelling that mines the era for more than its events. Is your story layered? Should it be?