Lack of knowledge..
No doubt, achieving success is hard work. It takes willpower and motivation to accomplish goals.
In other words, the more you demand of yourself (within reason, of course), the more you'll achieve.
Lack of focus and persistence! For many, if they don't have immediate success at something, they usually fall prey to the next new thing.
The simple answer is,
The #1 thing that holds people back from achieving their goals is...not taking action.