This is known as reciprocity failure of the material (film, paper, or sensor).

He calls this the reciprocity failure of light fading.

If so, you may have been a victim of what's known as reciprocity failure.

Moreover, they take no account of color shifts or reciprocity failure.

However, this relationship breaks down outside these limits, a phenomenon known as reciprocity failure.

Some films are very susceptible to reciprocity failure, and others much less so.

Did they think that reciprocity failure was a bad relationship between the subject and the photographer?

This is less true for extremely long or short exposures, where we have reciprocity failure.

The external links cited provide guidance on how to compensate for reciprocity failure.

Digital camera image sensors can also be subject to a form of reciprocity failure.