Why would a new brake booster not work when installed?

A new brake booster may not work if it is improperly installed, such as misalignment with the master cylinder or incorrect hose connections. Additionally, if there are leaks in the vacuum lines or the booster itself is defective, it can prevent proper function. Other factors like a malfunctioning check valve or issues with the brake pedal linkage could also contribute to the failure. It's essential to ensure all components are correctly installed and functioning for optimal performance.