Why is it so difficult for the jury in 'Twelve Angry Men' to reach its final verdict?

I think thew jurors really had to get past their personal bias and prejudice. Looking at prejudice in a larger sense, we find that, while maybe not racially driven, many of the jurors enter the jury room with preconceived notions and irrational ideas. 3rd Juror seems to be prejudiced against the accused simply because of his age, which seems to remind him of his estranged son. An interesting example of "reverse prejudice" is 8th Juror, who is initially sympathetic to the accused, not because of the evidence, but because he pitied his poor and troubled upbringing.