Let's assume the following extremely simplified equation is true:

CALORIES_IN = WORK + FAT

Usually the conclusion is "less calories = less fat". But it also could be "less calories = less work". Not just in the sense that you consciously decide to work less, but also that your body can make you unable to work. Which means: you are extremely tired, unable to focus, in worst case you fall into coma.

The problem with calorie restriction is that it doesn't come with a switch for "please don't make me tired or falling in coma, just reduce my fat". -- Finding the switch is the whole problem.

If your metabolical switch is broken, calorie restriction can simply send you in a zombie mode, and your weight remains the same.