If any of you have trained fat people you know it is very difficult for them to lose much fat. Is it any wonder that most of them give up after a short time? The fact is most people quit going to the gym after a month or two.
Which is why they fail.

You ever have a client who followed your instructions with 100% compliance, never quit, and failed to lose any fat? I doubt it (unless your instructions sucked or they lied to you about their compliance), since I've never seen anyone maintain their weight on a hypocaloric diet.
Is it harder for some than for others due to genetics, age, or medical conditions? Sure, of course. But ultimately, the reason people fail is they cheat and/or quit, usually both.
Hope this helps.