If recipient’s email platform doesn't display images. Some email clients display images by default, and others ask for permissions to display images. In case client don't allow permissions for image display then email tracking can't work.
If recipient’s using plain text email.Since email tracking mechanism puts an image inside email to track information, so plain text emails can not be tracked.
If email is having multiple recipients, then system can't find out exact information about who opened the email.
If recipient is using some proxy server, then its hard to find out actual location of user, and we can only identify location of that proxy server.
When recipient is using Gmail app for email viewing. Gmail has automatically download images using proxy to obscure the end-user and provide a layer of anonymity. So in this case we can always identify if email was opened or not, but its location will not be tracked properly. You can find Gmail's official statement over here.