Hi!!
Marketo Predictive Content uses machine learning algorithms to analyze user interactions and behavioral data, creating a profile for each recipient based in it's demographics and behavior data.. The algorithm scores and ranks content based on relevance to the recipient's profile. When sending the email, the highest-scoring content is dynamically selected for each recipient, ensuring personalized and engaging content delivery. Marketo is continuously learns and refines its predictions with each interaction.
When you are doing a preview or sending samples to test it, it wont work as when sending a sample to any email it wil choose a random option of your predictive content, not based in the ML Algorithm. For preview image as @sanfordwhiteman said it's the same scenario