I guess it may happen because of the idea being implemented in the wrong way, or because it doesn`t fit the current project.
Some idear are good but have a poor timing. In those cases i save the idea for a better ocassion. This could be another point on the story, or for a different comic/novel.
For example, i had some character ideas and designs i loved, but realized they don`t fit my current comic, so i save that for another project.