--1) job_queue_processes may be too low (this is the most common problem)
--The value of job_queue_processes limits the total number of dbms_scheduler
--and dbms_job jobs that can be running at a given time.
--To check whether this is the case check the current value of
--job_queue_processes with

select value from v$parameter where name='job_queue_processes';

--Then check the number of running jobs

select count(*) from dba_scheduler_running_jobs;
select count(*) from dba_jobs_running;

--If this is the problem you can increase the parameter using
alter system set job_queue_processes=200;

--2) max_job_slave_processes may be too low
--If this parameter is not NULL then it limits how many dbms_scheduler jobs can
--    be running at a time. To check whether this is the problem,  check the current
--    value using

select value from dba_scheduler_global_attribute
    where attribute_name='MAX_JOB_SLAVE_PROCESSES';

-- Then check the number of running jobs

select count(*) from dba_scheduler_running_jobs;

--If this is the problem you can increase the number or just NULL it out using

exec dbms_scheduler.set_scheduler_attribute('max_job_slave_processes', null)

--3) sessions may be too low
--This parameter limits the number of sessions at any time. Every Scheduler job
--requires 2 sessions. To check whether this is the problem,  check the current
--valule using

select value from v$parameter where name='sessions';

--Then check the current number of sessions using

select count(*) from v$session ;

--If the numbers are too close you can increase the maximum using
alter system set job_queue_processes=400;
--4) Have you recently applied a timezone update patch or upgraded the database
--to a version with newer timezone information ? If you skipped any steps when
--updating the timezone information,  jobs may not run. To check whether this
--is the case try doing

select * from sys.scheduler$_job;

--and

select * from sys.scheduler$_window;

--and make sure they finish without errors.
--If it throws a timezone warning,  reapply the upgrade or
--timezone patch making sure to follow all the steps.

--9) Has the Scheduler been disabled ? This is not a supported action
--but it is possible that someone has done it anyway. To check this do

select value from dba_scheduler_global_attribute where attribute_name='SCHEDULER_DISABLED'

--If this query returns TRUE then you can fix this using

 exec dbms_scheduler.set_scheduler_attribute('scheduler_disabled', 'false');

--6) If a simple run-once job doesn't run,  you can try restarting the scheduler as follows.
--- start here --------
exec dbms_scheduler.set_scheduler_attribute('SCHEDULER_DISABLED',  'TRUE');
alter system set job_queue_processes=0;
exec dbms_ijob.set_enabled(FALSE);

alter system flush shared_pool;
alter system flush shared_pool;

exec dbms_ijob.set_enabled(TRUE);
alter system set job_queue_processes=400;
exec dbms_scheduler.set_scheduler_attribute('SCHEDULER_DISABLED',  'FALSE');

----stop here --------