Why would the data for a Search Network campaign show conversions but no view-through conversions?

• A view-through conversion is counted when someone sees an ad in Google Search and calls the business
• A view-through conversion is counted when someone clicks on an ad in Google Search and converts on the site
• A view-through conversion is counted when someone sees an image or rich media ad on the Display Network but doesn’t click, and later converts on the site
• A view-through conversion is counted when someone clicks on an image or rich media ad on the Display Network and converts on the site