A) if you are really under 20g carbs, then it should be effective. I'm not sure what you're doing wrong. If a lot of your diet is cheap protein shakes, then they can have a lot of unaccounted for sugars.
B) Extended cardio is a horrible idea if you are under 20g carbs, and is probably not the best way to reach your goals (unless your goal is to be able to run for a long time, running for a long time isn't really a great workout). Instead, short intense workouts that increase your resting metabolism is probably a better option. High rep weight training, for instance, with a bit of short, light cardio (under 30 mins, low heart rate) every morning.
EDIT: I would recommend following the
V-diet training programme. It was put together specifically for doing low carb (even lower than <20g) and should be very suitable for you. Just pick one that fits your experience level, and go from there. Be sure to add the very low intensity walks each morning before eating.
That's what I did before, going from 200 lbs or so down to 186 in just over a month. And I'll be doing it again to go from (hopefully) 220 down to 200
