Potentially a lot of things. Are you targeting the same people? Are there more than 200,000 people in your target group? What is the reach of your campaign? Has most of your target group already seen your ad at least three times? Can you set up two landing pages for opt-in content, and split-test them? Have you tried setting up a new campaign with new images, headlines, and ad copy, but with the same landing page? Conversion is a complex process with many factors. It’s important to dissect as much as possible to understand what is going wrong. New campaigns can cure many ills. On the other hand, maybe the whole thing was a coincidence, and you’ve already reached all of the people who are willing to participate in your target audience.