How did a prospect reply or click my campaign but not register as an "open"?

Open tracking is triggered when a prospect receives an email and opens an email, however the tracking itself is in a pixel. If the prospect doesn't download images for their emails like  this, the pixel won't be triggered and the open will not register in Sendbloom even though they may be opening your email (as evidenced by replying or clicking!)

These settings are set in their email client (ie Gmail, Outlook, etc.) and are just a preference for that particular prospect.