The Yalu jailbreak, just like the Pangu jailbreak before it, relies on certificates that have expiration dates. You have to re-sign the jailbreak with Cydia Impactor past the expiration date to keep your jailbroken state.
No matter what kind of certificate you have, whether it’s a regular 7-day certificate or a 1-year developer certificate, it won't last forever, and a new jailbreak app called CertRemainTime by lululombard can help you keep track of it.