Different bodies, in the same way as you may have a bigger or smaller nose, deal with calorie excess and loss in different ways. Some bodies will deal with a deficit by going straight to 'BURN THE FAT!!', and others will deal with the same deficit by cutting other functions to run at less than optimal. Obviously, at some point, all bodies will lose fat at a sustained (but ultimately unsustainable) level of high deficit, but many people cut calories by a couple of hundred a day, as per many dieting recommendations, and don't lose weight at all. They do feel crap though (anxiety/poor skin condition/sleeplessness/mood swings etc), an that's because their body is dealing with the calorie deficit by being a bit lax on homeostasis.
This is never explained by WW and the like. They just make the assumption that all bodies are the same, and cutting your calories will lose your fat. It works - for some. The fact is, our bodies don't want to lose fat. Fat is the body's savings account, to fall back on in hard times. People who keep their fat despite multiple dieting attempts would last longest in a famine. In terms of evolution, it's way better to have lank hair, spots, and be a bit short of sleep than it is to be short on fat.
This is why some people remain overweight despite trying very very hard to lose fat. I'm sure some are lazy, and some are continually eating too much, but that's not the whole story. I wish that this process was explained more commonly, because I've got a few friends who have become very frustrated over the years with desperately dieting and feeling worse and worse, with no weight loss. The feeling of being defeated adds to the feeling of low energy, and really, at that point, I can understand why the only thing that would make you feel better would be to say 'Fuck it!' and have 3 portions of cake.
The way around it is to know how to encourage your body to burn as much fat as it is able. It still will be slower for some than others, but everybody sees progress at their own rate.