Shown below is a typical Kieninger grandfather unit. It may be a KSU or MSU Type but the action is all the same. On occasion, especially after some wear occurs, the locking lever does not lift high enough to unlock from the locking cam, or it may not lift high enough for the strike rack teeth to clear the strike locking lever. Sometimes the setting from the factory is not quite right in the first place. There is a remedy for that.

Shown above is a closeup of the chime locking cam, the quarter chime cam, and the locking lever. Circled is a tab that rides on top of the quarter chime cam. This can be bent down slightly so that it rides higher on the quarter chime cam. That in turn causes the lever to which it is attached to raise higher to clear the locking cam.

This lever extends leftward and on the 4th quarter raises the strike locking lever. As a result the strike locking lever will raise higher as well and the strike rack will fall freely.

That should solve your problem.