Only the user who locked the page, or the “admin” user, can unlock it. Members of the administrators group, apart from the “admin” user, cannot unlock pages which were locked by other users.
The documentation (http://dev.day.com/docs/en/cq/current/wcm/page_publish.html#Locking%20Pages) states:
You can only unlock locked pages if you locked the page or if you have administrator privileges.
This may be misleading, as it is not sufficient to have administrator privileges (i.e. general members of the administrators group). Only the “admin” account has the correct privileges.
How to unlock a page
It is possible to unlock a page using the sidekick in WCM, or using Content Explorer.
1. Using the sidekick
Log into CQ as the user who locked the page or the “admin” user.
Open the page that is currently locked in the WCM Websites panel.
In the sidekick, select the Page tab and then “Unlock Page”.
2. Using Content Explorer (http://<server>:<port>/crx/explorer)
Browse the tree to the locked page.
Right-click on the jcr:content node below the locked page.
Select “Unlock” from the context menu.