To lose weight you have to burn more energy than you ingest - fact. However, although CICO always applies (law of physics), it tells us nothing about why you are burning x amount of energy. It is much more complicated than simply eating less cals. (thread)

Under-recognized causes of weight loss difficulty:
hormonal imbalances/changes, i.e. PCOS, menopause, low estrogen, low T in men, etc.
hypothyroidism
meds
stress (cortisol, insulin resistance, inflammation, etc.)

inflammation, i.e. autoimmune disease, histamine intolerance, individual food sensitivities, etc.
lack of sleep (hormones: insulin resistance, leptin, ghrelin)

adaptive thermogenesis: cycles of weight loss and gain naturally slows down metabolism, eating too few calories for long period of time
nutritional deficiencies: B vitamins, magnesium, iron, protein, etc.
impaired gut health
leptin resistance