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The queue triggered event job only when the enqueue occurs. Moreover, if a new enqueue occurs during the job execution, the scheduler will skip that event.
Simplest way is to start a single non-event job with a continuous dequeuing. Or enable multiple event job instances and dequeue all messages on startup. The third option is to register the pl/sql callback, but you still need to read messages on startup. Pl/sql callback is starting up to 20 processes on simultaneous enqueue.

i enable this one as well, still ,it is not working.
DBMS_SCHEDULER.SET_ATTRIBUTE('<<jobname>>','parallel_instances',TRUE);
As per your comment " enable multiple event job instances and dequeue all messages on startup. "
do you have sample,please share it?

Parallel_instances is needed to solve this problem_:_
Note:
The Scheduler runs the event-based job for each occurrence of an event that matches event_condition. However, by default, events that occur while the job is already running are ignored; the event gets consumed, but does not trigger another run of the job. Beginning in Oracle Database 11g Release 1 (11.1), you can change this default behavior by setting the job attribute PARALLEL_INSTANCES to TRUE. In this case, an instance of the job is started for every instance of the event, and all job instances are lightweight jobs. See the SET_ATTRIBUTE procedure in Oracle Database PL/SQL Packages and Types Reference for details.
How to dequeue messages from queue, you can find in the documentation Advanced Queuing User's Guide (pl/sql). See also Administrators Guide about event based jobs.

i tried with parallel instance,it is not working.still event are the in que table.