You should understand that the lemon detox diet is a great way to get rid of unwanted toxins, and not for weight loss. Perhaps, it is just a consequence because when you are doing a master cleanse, you only feed on liquids. However, when you reduce your intake of calorie, your body learns how to get by with less fuel so your metabolism slows down.
With slower metabolism, you may feel weaker. Thus, to be clear, you should remember that the primary goal of a lemon detox diet is to let your digestive system take a break, so that your body would have the opportunity to retune itself while you stay energetic at the same time.
If your goal is to lose weight rather than to eliminate toxins from your body, you will find yourself gaining weight right after your detox diet. This is because since your metabolism had slowed down during the diet, it would continue to slow down even after the diet. Thus, as you go back to your regular eating habit, your body would have a slower pace to burn off the calories that you are taking in.
In summary, you should consider the lemon detox diet as a detoxification regimen for your body at least twice a year. Purging toxins and unhealthy chemicals from your body should be your ultimate goal and not losing weight. Though losing weight is an offshoot of this diet, it should not be your main purpose.
If you really want to lose weight, you should try to exercise regularly and eat a balanced healthy meal.