Why would a Yale key suddenly stop working?

I have a Yale key for my lobby door and I thought I'd make a few copies for emergencies - but the copies now don't work. They were cut correctly because they've worked before but I tried them this morning and they wouldn't turn in the lock. The original still works perfectly. Any ideas?

What is a VPN?

VPN is (Virtual Private Network) were first used by companies to enable their employees to securely access internal systems such as email remotely (e.g. from home or while on business trips). Today they are increasingly being used for personal use by individuals to protect their privacy while online in public places (e.g. when using the wi-fi connection in a cafe) or in a country where the internet is censored / blocked (e.g. China, Saudi Arabia…).