Someone who's unperceptive doesn't notice or understand things. An unperceptive book review misses the entire point of a great novel.
When you perceive something, you see it or comprehend it, and if you're good at that, you're perceptive. Perceptive people are sensitive and able to see even what's only hinted at. Unperceptive people are generally unobservant, especially when it comes to subtle details. An unperceptive reporter will fail to ask the right questions, and an unperceptive detective will miss many clues.