"I eat almost nothing and don't lose weight"
"I eat less than everyone around me and still don't lose. Is something wrong with my metabolism?" This page is written with respect, because the feeling is real, it is common, and the people who have it are usually trying harder than anyone gives them credit for.
What measurement finds
The question has been tested directly. In a well-known study, people who reported eating very little while failing to lose weight were measured with the doubly labelled water method — the reference technique, which tracks expenditure without relying on anyone's memory — and their food intake and activity were compared with what they had recorded. On average, intake was substantially higher than reported and activity substantially lower; their measured metabolism was normal for their size. The group with this specific "resistance" in that study was small — ten people — so the size of the gap is not a number to apply to yourself. The direction has been found again and again, in many populations.
Read that carefully, because it is not an accusation. Nobody in the study was lying. Memory for food is poor, portions are misjudged in a consistent direction, cooking tastes and drinks and "just a bite" do not register as eating, and the error grows with weight and with the length of the diet. It is a systematic error of the instrument — a human remembering — not of the person.
The rare real causes
Some things genuinely lower expenditure or raise weight independent of food: an underactive thyroid, certain medications (some antidepressants, antipsychotics, steroids, some diabetes drugs), very low lean mass after long restriction. These are real, they are diagnosable, and they belong to a doctor with a blood panel — not to a stricter diet. If the honest week below shows intake truly at or below your estimated expenditure and the trend is still flat, that is the appointment to make.
The one honest week
One week, with a food scale, weighing everything that goes in — including oil, drinks, sauces and what you taste while cooking — and logging it at the moment, not from memory that evening. Not forever; one week. It is the only way to answer the question for yourself instead of arguing with a website, and most people who do it find the answer in the log before the week is over.
Compare the week's average with the daily energy calculator. If intake was well under the estimate and the trend has not moved for a month, see a doctor. If it was not, you have not failed — you have found what to change, and the tracking step shows how to keep it up without it taking over your life.
In the app
The diary takes free text alongside "as planned" logging, so a weighing week is possible without a second app, and the trend on the progress page is what you compare it against.