Drink a green tea or any other herbal tea twice or thrice a day.Drink at least 4-5 liters of water and exercise regularly.
Have you had your thyroid levels tested to make sure you aren't hypothyroid? Weight gain/inability to lose is one of the main symptoms of being hypo.
Beyond that, it's possible that 1400 calories is not enough.
You say that you only eat 1400 cal a day. What is your carb intake, Healthy Fat intake and Protein intake? How many meals is the 1400 cal split up over and what is the intensity that you do as cardio?
To be honest just running is not always the best way to burn Calories because your body gets used to it! Interval cardio is usually what I put all of my clients on because it is more intense! Do you do Cardio before or after your workout? What time do you workout and do cardio? Do you do cardio on an empty stomach First thing in the morning to attack fat storage? There is so many things that you could be doing wrong and franly haven't provided the proper information for someone to give you the proper advice!! If you could be a little more specific maybe I could help you reach your fitness goals!
You may not be getting enough cals for how much your working out, plus when you are smaller its harder to lose cus you dont have much 2 lose you just have to keep pushing and it will come maybe not as fast as you want but it will