The Cross-Sell recommendation box will only show items you've manually defined them in the Upsell & Cross-sell. If you want to learn more about how to manually define cross-sell items with the upsell & cross-sell function, check out this article.
However, you may select a fallback option for your Cross-sell box. The fallback option will allow the Cross-sell/Up-sell box to default to another automatically generated list of products when there is no Cross-sell/Upsell connection for a product. To learn more about Cross-sell/Up-sell fallback options, please click HERE.