This can typically occur in a scenario if the recipients have some kind of blocker on their end or if they do not click 'always display eternal images' in their emails. Because tracking is done with a pixel tracker, if images are disabled on the recipient's end it won't be able to track.
Certain email clients will only display images if their settings are turned on, so if images aren't displayed this affects tracking, but only the recipient has control over this.
There could also be a scenario for example: wherein you have a 'URL' that has been added to a manually created 'Custom Field' called 'Meeting Link' and the custom field has been entered into the email template via 'Tag' as shown below.
In this case, you would want to right-click on your inserted 'Tag' within the template and click 'Link'. A popup will appear where you will see the below information added in the 'Text to display' field which has come from the custom field created property,
But for the link to be trackable, the hyperlink needs to be present in the URL text box. All you need to do is copy the content from the 'Text to display' field and paste it into the URL text box and click 'Save'. In doing so that property will create a URL and hyperlink it in our system when it sends, thus making it trackable.