It no longer assays to the label
Updated
A tub can look perfect, smell perfect, mix perfectly and contain considerably less of its active than the panel says. Nothing about it announces this.
The failure with no symptom
Potency is the only attribute of a supplement that cannot be assessed by any means available to the customer or, without testing, to you.
Which is why it is the failure most likely to persist unnoticed across an entire production run and a year of sales.
Four routes
It was never there. The raw material was not what it claimed, or assayed below its own specification. Identity and potency testing at receipt is what catches this.
It was not distributed evenly. The blend is non-uniform, so the average is right and the servings are not.
It degraded. Oxidation and moisture, over shelf life, faster in warmth. Sensitive vitamins and botanical actives decline; minerals largely do not.
It was formulated to the claim rather than above it. Without overage, a product that assays exactly at claim on day one assays below it later.
Why it is a labeling problem
The Supplement Facts panel states an amount per serving. That is a quantitative claim, and a product that does not meet it is misbranded rather than merely disappointing.
It is also testable by anybody, including a competitor, a journalist or a regulator, which is a different exposure from most small-manufacturer categories.
What to do
Test finished potency on the first batch of every formula and on any change. Formulate with a deliberate overage sized to expected decline. Protect the product with a real moisture barrier. And set the shelf life from evidence rather than from convention.
Then keep the records, because the question that eventually arrives is what this batch assayed at and what you did about it.