There is very likely nothing wrong with your body. It's just that some people lose weight more easily than others for a variety of reasons. There are some points you might want to consider though. First, if you are only having one meal per day, and fasting the rest of the time, you are not doing 16:8 IF. You are probably doing something like 20:4 or even 22:2. Going for such a long time without eating every day will cause more muscle loss than if you had a larger eating window. Also, if you are not having any carbs at all, your metabolism will start to slow down after a while, and certain hormones will get out of balance. This is particularly the case if you are quite active. So I'd recommend you have at least one meal per week where you include some starchy carbs, and you can increase that when you have lost more weight. Also, make sure you get out for a walk most days of the week, and ideally lift some weights (or do some callisthenics) at least two days per week as well, assuming you are not already doing this of course.