Answer
Check three things in order: whether another background job is already executing in that same queue (which can block subsequent jobs from progressing); whether the Default batch queue is actually enabled; and whether the database's JOB_QUEUE_PROCESSES parameter has been set to 0 (query select value from v$parameter where name = 'job_queue_processes', and if it's 0, raise it with ALTER SYSTEM SET JOB_QUEUE_PROCESSES = <value>) — any of these three can independently cause jobs to sit stuck in Posted/POST status instead of progressing automatically to execution.