The developer was dead

What oxidation does

The developing agents are consumed by oxygen rather than by silver halide, so the working strength falls. Development is slower and contrast is lower, and nothing about the liquid announces it.

So the diagnosis goes wrong

A thin negative reads as underexposure or as a metering problem, and the next roll gets more exposure rather than fresh developer. That is the real cost of a slow packaging failure.

Headspace is the cause

A bottle half full of stock developer has a bottle half full of air against it. The remedy is to eliminate the air rather than to seal it better, which is why the format matters more than the closure here.

Decant into small bottles, filled full

132 of the 191 containers on this shelf are at or under 500 mL. Splitting a stock solution across several of them, each filled to the neck, is the practice, and it costs a few bottles.

Date them

Because the failure is time plus air and the only way to know which bottle is oldest is to have written it on. A decant with no date is a bottle you will eventually distrust and discard unopened.

Keep a control if you develop regularly

A known film and a known exposure, run occasionally, tells you whether the chemistry has moved. That is the only way to separate a chemistry problem from a technique problem.

Fixer does not behave this way

It is not oxidation-limited, so a larger part-full bottle is acceptable. The two chemistries want different strategies and treating them alike is how one of them gets wasted.

The misdiagnosis is the expensive part

A thin negative blamed on exposure leads to changed technique rather than changed chemistry, so the error compounds across rolls. Dating the decants is what makes the chemistry a checkable variable.

References