Since we know value investing works (e.g. low P/B and low P/E portfolios outperform), why might you, a value investor, be underperforming? In a fascinating series of posts, Greenbackd breaks down a paper that seeks to examine why different types of value investors underperform. For example, those who adhere to strict screens may underperform because of an unfortunate time period (for certain periods, high P/E and high P/B can outperform) or a lack of diversification.
Or, if you want more details, you can find the whole paper here.