The listener is not paying attention due to lack of interest in the subject matter. The listener is preoccupied with other matters. The speaker is not saying what the listener wants to hear. The listener is busy formulating a rebuttal to something the speaker said previously. Most miscommunications are the result of poor listening, not poor articulation.
Most people are passive listeners.
Even when information is received and understood, the listener fails to acknowledge it. Neither do they verify that the information received is what the speaker intended to say. Passive listening also allows misunderstandings to go undetected. The communication process consists of speaking and listening. Some people think that when you are speaking, you are in control of the process, and when you are listening, you relinquish control to the other party.