The preservative did not hold
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The preservative is on the label, at the recommended percentage, from a reputable supplier, and the product grew something anyway. This is common and it is almost always a system failure rather than an ingredient failure.
Present is not the same as working
Most preservatives are active only within a range of conditions. Outside that range they are still in the formula, still listed, still costing money, and doing nothing.
So the presence of a preservative on an ingredient list tells you very little about whether a product is preserved.
The four reasons
pH is outside the active range. A common cause. Organic acid preservatives depend on being undissociated, which is a pH function, and a formula adjusted for skin comfort can sit outside where the preservative works.
Water activity is too high for the level used, so the system is being asked to do more work than it was designed for.
No chelator, so trace metal ions are available to the organisms and the preservative is fighting uphill.
The package exposes the product more than the design assumed: an open jar instead of a pump, fingers and air on every use.
How you find out
A challenge test, before launch, on the final formula in the final package. The alternative is finding out from a customer, which is the same information arriving in the worst possible form.
Counts at days 0, 2, 7, 14 and 28 show not only whether the system killed what was introduced but whether anything recovered afterwards, which is the failure mode a short test would miss.
The repair that is not more preservative
Raising the dose is the instinct and it is usually the worst of the available options: it costs on every unit, it raises sensitivity risk, and high preservative loads dehydrate emulsifier chains and can split the emulsion you were trying to protect.
Correct the pH into the preservative's active range. Lower the water activity. Add or increase a chelator. Change the package to one that exposes the product less. Then retest, changing one thing at a time so you learn which repair worked.