General Options ->Sources -> Search in custom fields
In some cases you might need to search in custom fields as well, as some custom post types store data in custom fields.
Searching within specific fields
Use the search bar to look for the desired fields
Drag & Drop the fields needed from the left side of the list to the right
Searching all fields In cases where most of the fields are required for search, it is easier to simply enable the Search all custom fields? option.