The documentation says:
The mixed booking allocations are left unprocessed in the following cases:
1) A scheduler has locked the mixed booking allocations for a project.
2) The user selects the target booking status as Mixed instead of Soft or Hard.
Considering that point 2 was correctly set in the job detail, remains point 1: A scheduler has a lock on a project.
To check and resolve this, go to:
Here the possible import/export locks are displayed.
After removing the locks, the job should run successfully and not leave unprocessed records anymore.