Many times we ask ourselves this question, well here are some of these reasons:
Your diet is based on processed foods.
You exceed the consumption of healthy foods, but high in calories. Like nuts for example.
You are NOT consuming enough foods rich in fiber and protein.
The water intake that you are having is insufficient.
You exceed the consumption of oil through preparations and/or salads.
You let too many hours go by without eating, thinking that this way you advance faster.
You overestimate the calories and macronutrients you eat during the day.
You don't get enough rest.
Regarding physical activity; if you focus clearly on doing cardiovascular exercises. You do not give emphasis to resistance exercises and weights.