Crontab not triggering scheduled job

Question

I have a Java program that reads text files and processes them sequentially and on completion of each file a mail is triggered in a separate thread (using runnable). I have configured this program as a cron job in crontab to run every 15 mins.

The cron does not get triggered if the child thread of the last job run takes more than 20 minutes to send the email. How can I have the job triggered every 15 minutes, even if the last child thread is still executing?

Below is the code used::

EpdFeedLoader.java

public class EpdFeedLoader {
    public static void main(String[] args) throws Exception {
        startProcess(feedFile);
    }

    public static void startProcess(BatchFeedFile feedFile) {
        EpdBatchLoader epdBatchLoader = new EpdBatchLoader(feedFile);
        epdBatchLoader.execute();
    }
}

EpdBatchLoader.java

public class EpdBatchLoader {
    private static void sendDelayedFeedEmail(){
        SendDelayedFeedJobStatusMail delayedFeedEmail = new SendDelayedFeedJobStatusMail();
        delayedFeedEmail.setParameters(//parameters for my email method);
        Thread delayedFeedThread =  new Thread(delayedFeedEmail);
        delayedFeedThread.start();
    }
}

SendDelayedFeedJobStatusMail.java

public class SendDelayedFeedJobStatusMail implements Runnable {

    private MailRequestDTO mailRequestDTO;

    public void setParameters(MailRequestDTO mailRequestDTO){
        this.mailRequestDTO = mailRequestDTO;
    }

    public void sendMail() {
        boolean isSend = false;
        message.setSubject(subject);
        message.setSentDate(Utils.getDateInTimeZone(mailRequestDTO.getCountryCode(),new Date()));
        message.setContent(mailRequestDTO.getMessageContent(), "text/html");
        message.addHeader("Content-Transfer-Encoding", "7bit");
        attachInlineImages(message, mailRequestDTO.getInlineImageMap()); 
        ConfigUtils.logger.info("Ready to Send Delayed Feed Receipt Email for Process Code : " +  mailRequestDTO.getProcessCode().toString());
        Transport.send(message);
        isSend = true;
        ConfigUtils.logger.info("Delayed Feed Receipt Email Sent Successfully for Process Code : " +  mailRequestDTO.getProcessCode().toString());
    }   

    @Override
    public void run(){
        this.sendMail();
    }
}