It lacked depth.

Meaning

The work failed to explore themes or characters sufficiently.

Examples

• John: I found the movie a bit underwhelming.
• Alice: Me too. <b>It lacked depth</b>. The characters weren't very well-developed.
• John: And the plot felt a bit superficial.
• Alice: Yes, it's true. In summary, <b>it lacked depth</b>.