People are preoccupied with family concerns, other work, pressing assignments, relationships, transfer, etc. Differences in personalities. If someone doesn't like you, they may decide not to listen to your message, thoughts, or ideas. As a leader, you need to watch and be aware of the expressions of those in the audience to be sure they are taking in what is being said. Once again, this sounds easy, but think of all the pitfalls.
Language is imperfect.
Semantics, slang, inflections, jargon, or a play on words can easily change the meaning of a message. At least half of all communication time is spent listening. A leader must also communicate an aura of approachability. At work, all of us spend countless hours involved in listening-related activities. Listening is both a behavior and a skill.