Why will a new barrel and key not work?

A new barrel and key may not work if they are not properly matched or compatible with the existing lock mechanism. Factors such as incorrect specifications, misalignment, or a lack of proper installation can prevent the key from turning or engaging the lock. Additionally, if the lock itself is damaged or malfunctioning, even a new key and barrel won't resolve the issue. Lastly, if the lock has been rekeyed or altered previously, a new set may not correspond to its internal components.