There can be multiple causes. Theme modifications, apps, and manual installs. The later isn't your problem judging by previous posts. You can test the theme by doing preview on an unmodified theme. Testing apps is best done by creating a new shop and selectively importing your shop. That's how I was able to rule out everything and figure out that it was just Shopify's implementation of the FB Pixel.
The issue has been fixed on the Debut theme for myself and OnAMountain said its working on the Venue theme. I don't think its theme specific but if you're still experiencing the issue you should try your theme unmodified and then one of these.
|a minute ago|