Why was a work item not reassigned to the coverer?
Check three things in order: the coverer can be assigned work items in that space, the coverer is not out of office themselves without a coverer of their own (the app follows such a chain for up to 5 steps), and no Jira automation rule is assigning the work item back.
The rule's Reassignment Log lists every work item it could not move, with the error each hit.
Related pages
Issue Assignment and Watcher Management - where this is covered in full.
Troubleshooting - more questions on this topic.
