I run into a few people from time to time who tell me that low-carb did not work for them. Many times the reasons are related to things mentioned in this helpful article:

lowcarbdiets.about.com/o
d/lowcarb101/a/lowcarbmist
akes.htm

Many other times people have other health issues that keep them from being successful. Common problems are two that I have experienced in the past: hypothyroidism and other allergies that affect how comfortable and energetic one feels.

Another factor that can play a role are existing eating disorders, an unhealthy environment (family not on board, too much stress, too little sleep etc.) . These are all reasons why other steps need to be added to a low-carb diet (or any diet) to be successful.