It went thin
Updated
Customers read thickness as strength, which is why a cleaner that thins on the shelf reads as a cleaner that has gone weak.
Thickness is a claim
A thin cleaner runs off a vertical surface and is judged as watery. A thick one clings, dwells, and is judged as powerful.
The performance difference is real on a vertical surface and mostly perceptual elsewhere, and the perception is what sells.
Why it thins
Microbial attack on the thickener, which is the classic: a polymer or cellulose thickener is food, and viscosity loss is often the first sign of a preservative failure.
pH drift, because several thickening systems are pH-dependent and lose structure outside a band.
Salt, which thickens a surfactant system up to a point and thins it sharply past that point. A formula near the peak is unstable in both directions.
Heat, sometimes permanently.
Shear, from over-mixing during manufacture, which breaks polymer chains.
Why it thickens
The mirror problem and it is worse in the bottle: a product that gels will not pour, will not spray and will not come out of a trigger.
Usually cold, usually a formula near its solubility limit, and usually reversible on warming after the customer has already thrown it away.
Holding it
Pick a thickening system suited to the pH and stay inside its range.
Preserve properly, because a thinning product is often a microbiology result.
Stay off the salt peak: formulate on the low-salt side of it, where a small variation moves viscosity less.
Mix at the lowest shear that disperses, and do not over-mix.
And measure viscosity on every batch against a target, with a consistent method, at a consistent temperature.