Why are email opens not being detected?

First, if you haven't done so already, see "How does email open tracking work?"

Vocus.io might not receive email open signals for one of those reasons:

1. Auto-Image loading is disabled. By default, most email clients will automatically load images in the body of the received email. Sometimes users will opt to "disable" this behavior, and instead block all images. Doing so will block the tracking pixel from being loaded, and Vocus.io will not be able to detect the event. 
2. You are sending an email to yourself. By design, Vocus.io will block tracking pixels that are created by the user. This means, when you send an email to yourself (example, sent it to your personal email inbox), Vocus.io will detect that the tracking pixel is owned by you, and hence will not load it. If you are looking to test email tracking, we recommend you send an email to a colleague.