Usually when you go on a diet you will notice most weight loss at the beginning of the diet. After a while you reach a plateau and weight loss slows down on a diet. Initially a lot of the weight loss will be fluid and also as you get slimmer your metabolic rate will slow down.
The more weight you have to lose the easier it is initially as your metabolic rate and calorie requirement is greater the decreases on weight loss so the calorie deficit the diet creates drops and as consequence the amount of weight lost drops.
In order for weight loss to continue you need to gradually cut your calorie intake in relation to your size. In creasing physical activity will also help as the amount of calories burned will help you lose weight.