Decreases in activity
When our calories are low and we are losing weight at a steady pace our bodies eventually want to slow down that weight loss to ensure that we keep as much stored energy(fat) as possible in case of famine (going back to when our ancestors did not know when the next meal would be). In order to conserve as much energy as possible we will subconsciously not fidget as much, walk around less, stand less, and reduce activity in general. It may not sound like much but it can add up to several hundred calories per day.
Solution: Consciously make an effort to be more active and force yourself to expend calories. Take the stairs, walk to the store, tap your foot, and just move around. By being aware of the fact that your body is going to want to move less you can counter act this by increasing your activity on a conscious level.
The heavier the body is the more energy it requires to perform all activities. When we lose weight the body has an easier time performing these activities so it does not require as many calories. So, a person who weighed 250 lbs at the beginning of their weight loss journey may have needed 3000 calories per day to lose 1 lb per week due to his/her weight, but after he/she has lost 50 lbs the body may only need 2500 calories per day to perform the same amount of activity and lose 1 lb per week. Therefore, the amount of calories this person was eating to lose weight at 250 lbs is now too much to lose weight at 200 lbs.
Solution: Adaptation is inevitable, but by decreasing calories 250-500 per day when your scale weight has not decreased at all over several weeks you should return to losing .5-1 lb per week. Another option would be to simply increase exercise as this will burn more calories and allow you to eat the same amount of calories, but still continue weight loss.