What do you think are the biggest reasons why many fail to earn online

Hello, I have recently joined this forum and would like to ask a question of the members here. What do you think are the biggest reasons why many fail to earn online? I have been learning a great deal about internet marketing now for quite a few months but really want to earn a meaningful income from this in 2012 so am keen to find out why people fail - and avoid those pitfalls ... Thank you so much for having me here