If fixed for too long then fixer starts to attack the exposed part of the negative and eats the silver away. If not washed and dried properly then the negative may start to turn yellow and fade as some fixer is still working on the negative. Spots and drying marks may show up if not dried properly. Make sure that chemicals are fresh and not contaminated. Exhausted developer gives underdevelopment.
Exhausted fix gives greyish fog over negative.
In addition to the set characteristics of film, we can manipulate exposure and development in order to achieve a better negative under certain lighting conditions. Shadow detail is controlled by the amount of exposure given Highlights are controlled by exposure as well but mostly by development. As shadows influenced more by exposure rather than development, then if development time is increased to 15 minutes, the highlights will become denser but the shadows will not be effected. By increasing the development time, this changes the contrast of the negative, the difference between the shadow areas and the highlight areas. Conversely, shorter development time given, then shadows still form but highlights are less dense - therefore overall contrast is reduced.