What would cause a parking brake to not work?

A parking brake may fail to work due to several reasons, including worn or damaged brake cables, which can prevent proper engagement. Additionally, issues with the brake mechanism, such as corrosion or malfunctioning components, can hinder its functionality. Low brake fluid levels or a leak in the hydraulic system can also affect performance. Lastly, if the brake pads are excessively worn, they may not provide the necessary friction to hold the vehicle in place.