Often its just very simple changes in what you eat and when can have a major impact. Don't be mislead by all the wrong well meant advice from friends and family i.e. high fat / high sugar / etc. foods. If you know exactly what to eat and when you will get far better results (you will also help your health in the long term !)
All the figures quoted in magazines, websites, etc are meaningless if they don't apply to you!. A six-foot athlete, with a very physically demanding job in his late teens will need to consume significantly different amounts of calories to put on weight, then a 5-foot office worker who doesn't take part in any sport. The key is to find out exactly how much you need to eat to gain weight and then compare this to your current intake. It could be you are already eating enough but that other factors are slowing down your weight gain.