Group discussion is hindered for a variety of reasons, one being each juror's bias and the lack of respect for the jury process. The majority of men are eager to declare the guilty verdict and be on their way, especially on an incredibly hot day. Juror 8 is the only man that wants to discuss the case, testimony, and really study whether or not there is or is not reasonable doubt. He outrages jurors 3 and 10 by being the only one to vote 'not guilty' and force everyone to discuss the matter.
Remember, all twelve men are VERY different; you need to consider age, background, occupation, and overall personality. Reginald Rose does an amazing job of giving readers clues about each juror within their dialogue. Prejudices and bias cause lack of communication and heated arguments. Many men assume the boy's guilt because of the background information provided. He is a "bad kid" from a "bad neighborhood" so therefore he killed his father. Jurors 3 and 10 feel so confident in this mindset that they refuse to listen to reasoning from Juror 8 and eventually others as the vote begins to more toward 'not guilty.' Juror 8's persistence, level-headed way of talking, and lack of bias allows more room for discussion and the eventual decision.