A user is unable to see a particular Product on the Product Selection screen when...

 Potential Causes:

Component Checkbox: Products marked as Components are typically excluded from being directly selectable in the Product Selection screen unless part of a bundle .

Search Filter: Custom Search Filters on the Add Products button limit the display of products based on criteria such as product attributes or field values.

 Excluded Reasons:

Hidden Checkbox: This is generally used to hide products from the Quote Line Editor, not the Product Selection screen.

Custom Action Condition: While these can limit the Add Products button's visibility, they do not directly affect individual product visibility.

 Salesforce CPQ Reference Points:

Documentation on Product Selection Behavior and Search Filters confirms these behaviors ​.