To grab their attention again and again.
When showing the visitor to your web page a call to action that is always visible (floating, attached to the top or bottom, etc.), don’t just “leave it there,” but change it over and over again. You can make it disappear and then reappear, change its shape, color, change its position, add a countdown, etc. The triggers can be time, scrolling, clicking, etc.
You might already use auto email sequences to communicate with your subscriber. The way it’s being done is preprogrammed. You send the emails at a certain pace, regardless of when and if at all they read them. That is not optimal.
At the end of each email have a call to action asking them to help you with just one click, something like: “Did you find this email message useful or enjoyable? Help me by simply clicking YES or NO.” The click will tell you two things: 1. They finished reading it. 2. If they liked it. You can use both to dynamically trigger the next email and also decide which email to trigger based on their reply.