Unfortunately, due to the wide variety of email service providers, email tracking technology cannot detect every email open. This is a limitation that's true for every email tracking platform.
Specifically, email tracking requires a recipient's email client to download images, and many email clients, such as Outlook, have image downloading disabled by default.
RocketBolt will alert you every time it detects an email open, but it can't detect every email open. No platform can. In fact, despite what some might claim, using the only available technology for email tracking means no product can more accurately track email opens than any other product.