some submissions don't trigger emails

Our Submissions form is set up to send an email (including uploaded files as attachments) every time someone submits. Most of them do this, but occasionally some don't. I can't figure out why. 

Yesterday a guy tried twice and neither one sent an email to me. Yet this afternoon someone submitted an entry, and I immediately received an email like I'm supposed to. 

What is causing this intermittent trouble?

I checked your jotform http://www.jotform.us/form/22885701668161 and I noticed a mistake in the Notifier settings.

The "Sender E-mail" was left blank -- this can caused the email delivery to have issue as some email service will not be able to properly determine the actual sender email. And can potentially block the email as spam or junk email only.

Check also the user guide 

http://www.jotform.com/help/208-How-to-setup-email-alerts-to-prevent-email-bouncing-related-issues

Let us know if issue persist.

Thank you for your help, Jonathan- I made that change and hope things will work better now.