In the short term, this is because when you begin a slimming diet, in the first week or two you will lose several pounds of weight which are fluid, not fat. After this, weight loss will be mainly fat.
For long-term slimmers, the main reason is that when you lose weight, your metabolic rate gradually slows down simply because you are getting smaller. When you were overweight, you will have been eating more than average to maintain that weight. For example, a woman of 12 stone may well have been eating 2,500 calories or so a day rather than the ‘average’ of 1,940. All she needs to do is cut down to say, 1,750 calories a day for daily ‘deficit’ of 750 calories, which will result in a weekly weight loss of 1½ pounds.
At 10 stone, however, she will be a fairly average weight and a daily diet of 1,750 calories is only slightly under the normal calorie intake (1,940) for a woman of her weight. To continue with good weight-loss, a diet of, say, 1,250 calories a day would be more appropriate. That would result in a daily calorie deficit of 690 calories and a loss of just under 1½ pounds a week.
For weight-loss to continue at a steady rate throughout any diet, you need gradually to cut the calorie content of the diet accordingly. In practical terms, it is probably best to be happy with slower weight-loss as you near target-weight and to increase your exercise levels a little to compensate.
Other reasons why weight-loss may slow down are that you have reached a reasonable body weight (you may have been aiming too low: check your BMI) or that you have begun eating more calories. Also, weight does fluctuate from day to day and week to week for a variety of reasons, including hormones, fluid levels, etc. Most women in the week before a period won’t lose weight at all and may indeed put a little on; this will disappear within days of the start of menstruation.