MQ message listener

I have created MQ listener in listen mode. I have set the time interval for listener as 1 minute. If there are more than one message in the queue, it just picks up the first one. It does not pick up the rest of the messages. But listener service will continues to process any new messages coming into the queue.
How do i make sure that once i enable the listner service, it processes all the messages already existing in the queue.

Thanks in advance.