Jamestown nearly failed because the English settlers were not properly equipped or prepared to live long-term in the New World. Most people who initially came to Jamestown expected to build wealth quickly and easily. When this did not happen they were left ill prepared for the harsh conditions of winter and many early settlers died as a result.
Many of the new settlers were not used to hard labor which led to a lack of food. Because of the lack of water and food, most of the population were dead with only 61 of the original 500 surviving. For more information, check out http://www.ask.com/wiki/Jamestown,_Virginia.