If your BMR (basal metabolic rate) is over 1800 calories per day, that means that that is the estimated amount of calories you need just to sit around and rest and not do anything. Like if you laid in bed all day, your body would need for you to eat that 1800 calories.
FitBit and other apps like MFP and LoseIt that track calories and provide you a calorie goal for the day, use TDEE (total daily energy expenditure).
TDEE is your BMR (of 1800) plus an estimated amount of additional calories that your body needs to function depending on your fitness/exercise level.
For instance, my BMR is similar to yours - approximately 1800 calories (about 1830). With a moderate level of activity, my TDEE is about 2200 calories. So if I did a 500 calorie deficiet, I should be eating about 1700 calories per day. If I exercise more (which I do every MWF) then FitBit and LoseIt (which I use for my calorie tracking) will tell me I have more calories that I can/should eat to compensate for my energy level.
Yesterday for example, FitBit told my LoseIt app, that I burned 1146 additional calories above my TDEE with my workout. Due to that, I ate 2400 calories yesterday because I basically needed more calories due to my exercise/energy expenditure.
Usually I eat about 2100-2400 calories per day on my heavy exercise days. On the other days of the week, I eat 1600-1800 calories.
I have been losing a consistent 1-1.5lbs per week for the past 3 months, which is my goal.
You may not be eating enough for your body to let go of calories easily. If your BMR is 1800, you should be eating at least 1800 calories a day for the most part and more on days you exercise.
