I've lead the marketing team for 3 VC funded startups that all successfully acquired customers en mass, but along the way I've certainly observed a fair share of startups that failed due to inability to acquire customers fast enough. Here are some common reasons startups fail:

1. They incorrectly think their product is so good that the customers will just show up and pay
2. The product is "cool" but doesn't solve a pain that valued in the market
3. Users like the tool enough to beta it for free but not enough to actually pay for it
4. Company acquires customers but the lifetime value to customer acquisition cost isn't sustainable, or requires such scale that the company never gets there
5. Most companies waste a ton of money on Adwords until they realize they should hire an expert
6. Startups acquire traffic but can't convert it into repeat purchases or usage
7. The market just isn't big enough
8. There's another solution that is more compelling

There's a ton of other reasons too. If you're having trouble acquiring customers, feel free to give me a ring. Happy to chat any time.