My suggestion would be to return the new scale and use the money for a nice, high fat, low-carb, meal/treat. Then carry on as you have been with the broken scale since it is obviously working for you.

Why on earth would you freak out over a difference in numbers on two non-scientific, likely poorly calibrated, scales versus what your own eyes and clothing are telling you?

But to briefly answer your question. If we were to believe for a minute the "no weight loss for four weeks" was true (and it likely isn't for the probable reasons already offered), scales do not just weigh fat loss. They also weigh fluctuations in water, bone, organ, muscle, etceteras. If you *really* want to go on an emotional roller coaster, try weighing yourself everyday for two weeks. Then do it for a month so you realize it's normal to fluctuate sometimes 5-10 lbs a day/week, depending on your personal physiology.

If you Google "why the scale lies", you'll find numerous articles/blogs on the topic written from a pretty wide slice of different dietary eating plan adherers.