I would suggest you ask David directly via the "Get In Touch" page on the Sweet Poison web site: http://sweetpoison.com.au/?page_id=29
He has previously suggested going "low-carb" for a few weeks to overcome poor loss or unexpected gain in weight.
How do you know that you need to lose "at least 20kg" ? What is your BMI ? [ kg/height(m) squared].
If is is over 30 you are obese, if over 25 you are overweight. It should be between 20 and 25.
It would be a good idea to calculate your ideal weight. Just Google "ideal weight calculation". Here's the top of the list: http://www.halls.md/ideal-weight/body.htI
Next I suggest you keep a detailed record of EVERYTHING you ingest for one or two weeks, and calculate your daily intake of fructose. It shouldn't exceed 10 grams. You could be be getting it in hidden sugars especially in processed foods. This web site is very useful for determining sugar content of Australian foods:
http://www.foodstandards.gov.au/consumerinformation/nuttab2010/nuttab2010onlinesearchabledatabase/onlineversion.cfm?&action=alphabeticalList&letter=S
How much fruit do you eat? Fruit juice? Sauces, relishes etc??
Your friends' experience indicate that the system does work - so don't give up. You just need to find out why it's not working for you. Is it possible that some emotional issues are undermining your motivation? From your comments I guess not. But thought it worth a mention.
Do you exercise regularly? I know David reckons it's not necessary in order to lose weight. But maybe there are some individual variables which might contradict the generality. Cutting saturated fat from your diet may also be worth trying for the same reason..
I'm casting a wide net here in the hope that something may strike a cord and be helpful to you. I do hope that you keep persevering, and looking here for support.
JohnN