There are many factors that could hamper an effective communication. Some of those could be: social status (if you are talking to someone who has a more important status than yourself, it may happen that they are not interested in a communication with you), use of language (if you use inappropriate words, the conversation is not going to be successful), knowledge about the subject (if you don't know what you are talking about, the conversation is likely not going to continue), atmosphere/noise, cultural differences, disabilities, etc.