I'm in a calorie deficit but not losing weight — why?

Almost certainly because you are eating more than you think — not because your metabolism is broken.

This is the least popular answer in fitness, so let me show you the numbers rather than just assert it.

The two candidate explanations, weighed

Metabolic adaptation is real. Your body does burn fewer calories as you lose weight, beyond what the weight loss alone predicts. The well-controlled research puts this at roughly 30-100 kcal/day, and it tends to fade once your weight stabilises — which is precisely the state a plateau is.

Under-reporting is also real, and much bigger. People underestimate their intake by 12-20% on food recalls and 31-38% on food-frequency questionnaires. In a classic study, people who were certain they were "diet-resistant" turned out, under doubly-labelled-water measurement, to be eating far more than they reported. A few hundred calories of unlogged oil, snacks and weekend drift is a bigger number than adaptation ever produces.

The two effects are simply not the same order of magnitude.

Where the honest nuance lies

I don't want to be glib about this. The Biggest Loser follow-up found adaptation as large as ~500 kcal/day after extreme weight loss — and yet, tellingly, the contestants who kept the most weight off had the greatest adaptation. So adaptation didn't cause the regain. And plateaus do appear even in carefully monitored, high-adherence conditions, so "you're just lying about your food" isn't the whole story either.

Also: a stalled scale is not a stalled body. Water retention, glycogen and hormonal fluctuations can hide weeks of genuine fat loss.

What to do

Track honestly for two weeks — weigh things, count the oil, count the weekend. Most plateaus dissolve at that step.

And if you are tempted to slash calories further, know what the evidence says about that: aggressive deficits mainly cost you muscle when protein and resistance training are inadequate. One trial ran a ~40% deficit and the high-protein, lifting group still gained 1.2 kg of lean mass. Eat enough protein, keep lifting, and be patient before you cut deeper.