There are several reasons why an iPhone battery may stop working properly. Some of the most common reasons include:

Normal wear and tear: Over time, batteries lose capacity and may stop working properly.

Excessive use: If the iPhone is used for extended periods of time or if the battery is frequently charged and discharged, it can degrade faster.

Heat: Heat can adversely affect the battery, especially if iPhone is left in the sun or in a hot place for extended periods of time.

Use of power-hungry apps or services: Power-hungry apps, such as games or social networking, can reduce battery life.

Malware: If the device is infected with malware, it can increase battery consumption without you knowing it.

Factory problem or battery defect.

It is important to follow the manufacturer's recommendations to prolong battery life, such as avoiding charging the battery to 100% or leaving the device in a cool, dry place.

In general, one full charge and discharge cycle per month is recommended to maintain good battery performance.