9 stone sounds a little underweight. Your BMI works out at around 19.7 which puts you in the normal range.
Perhaps your body type dictates you will always be naturally lean and may never gain weight. You could be Endomorphic.
You first need to work out how many calories you are consuming daily, possibly you are not eating enough. The average woman needs 2000 calories per day to sustain current weight. Keep a food diary for a week charting everything you are eating, including calorie and fat content.
If ou find you are not eating enough, increase your calorific inake, particularly your carb consumption, be sure to consume some fats too, but preferably not too many foods high in saturated fat content.