Francis Pedraza has an incredibly insightful article on 3 reasons that Lean Startups fail. Below, I've summarized the three reasons, but there's a huge amount of build up and some advice for fixing these problems in the article.
1. Founders need to be more aggressive with expanding their ideas and monetizing them.
2. Founders don't understand how to do proper company building and fundraising.
3. There's not enough risk capital available in the market.
Go read the article. Francis finds that most of the Lean Startups he sees don't have a bad idea. It's all problems in execution.