If the Categories referred to here are the WordPress native category taxonomy, the queries for these term pages by default will only include the native post post type.
To add custom post types to category term queries, you can add a bit of code to your theme’s functions.php file, which modifies these queries via the pre_get_posts action to add your custom post type:
function wpa_category_custom_type( $query ) {
if ( $query->is_category() && $query->is_main_query() ) {
$query->set( 'post_type', array( 'post', 'questions' ) );
}
}
add_action( 'pre_get_posts', 'wpa_category_custom_type' );
The above assumes your post type is registered as questions.