Some people do not lose in a constant downward motion. When I am CCing and using my fitbit, I know that I should lose 2lb a week. Sometimes my scale gets "stuck" and bounces up or down around a certain weight for up to 10 days! It can be irritating to get on the scale and see these fluctuations, but I KNOW that if I keep on going, the scale will eventually give in, and I will have a nice woosh that takes a couple pounds with it.
Another thing is that I do not eat at a set calorie number. I eat within a calorie range of 1300 - 1800 calories, because there are some days that I am naturally hungrier. This last week I ate 9797 cals that averaged out to 1400 cals a day. Sometimes my weekly cals average out to about 1600 a day.