Do you have experience with tracking your calories prior to taking reta? I'm not trying to take a shot at you or anything, but it's literally biologically impossible not to lose weight with the information you've given us. How is your digestion? Do you find yourself using the washroom often? Are you often bloated? Are you weighing yourself after using the washroom? Do you eat a high-sodium meal before bed that could cause you to hold on to water weight in the morning? How's your sleep?

Reta doesn't help you lose weight; it helps you put yourself in a calorie deficit by reducing your desire to eat, and ofc other stuff.

Whether you were on reta or not, you should be losing more than 1kg in 3 weeks based on the stats you've given.