A. There are a number of factors in this. Biggest of them all is the fact that on most weight loss plans you aren’t really burning fat to begin with! There is only so much toxins and water you can lose, so inevitably results will stop. This will especially happen on what I call one dimensional plans such as those that have diet only and no exercise, meal replacements plans, pill only based plans and low carbohydrate plans.
Another key factor is that people lose focus. Human nature means that after 2 or 3 weeks of effort, we start cutting corners and taking short cuts. Often we do this without even realising it, we swear we are being as good as always, when truly we are not. We completely underestimate what it takes to keep losing weight.
The critical key to continuous weight loss is creating a deficit so that calories burned exceed calories consumed. In the beginning of a plan this can be easier when we are that bit heavier and changes we make to our food are radical to what we were on the weeks before. However to manage to keep maintaining this deficit so that results will continue is not easy.
Central to maintaining this deficit is to make sure that you are continually improving your metabolic rate and continually progressing your exercise level so that there is significant deficit to allow you see clear results. Unfortunately most weight loss plans do the opposite. They slow down your metabolism through under-eating and food imbalance and if you are lucky enough that they include exercise they rarely if ever make sure that you adjust it regularly to make progress.