There are various reasons why people say they can’t lose weight.  Usually it’s because they’re cheating and not really acknowledging their own bad habits.  But that being said, there are some actually legitimate cases that hold people back from losing weight.  This being said, most are not.  But the most common reason is basically you may have a thyroid disorder.  Often this is characterized by weight gain, inability to lose weight, constipation, depression, inability to handle cold temperatures, losing hair, no sex drive, agitation, moodiness, irregular menstrual cycles, muscle cramps, and general pallor of the skin. 

This is only with an underactive thyroid keep in mind, and it’s fairly rare still the same.  Frankly, most cases are rather minor and with a step up in work, you can still lose weight.  Others actually suffer metabolic syndrome, which leads to type 2 diabetes over time.  It generally shows itself by causing insulin resistance, obesity, abnormal lipids, and hypertension in many cases.  Again, these reasons are uncommon.  So if you are not losing weight, don’t automatically blame them, they are quite serious medical conditions. But at the same point, if you have other symptoms, it’s worth checking out.