Do you have either of these settings set to ON:

https://www.screencast.com/t/zmoArLmi

If you do, do you have the free Relevanssi plugin installed? You may only use either or both of those two settings if you DO have it installed. Indeed, if you do not have it installed, that may be the solution. Sometimes the theme used interferes with the WordPress text search.

Our plugin does not directly allow you to do text searches of individual Post Meta (custom fields) and Taxonomy (Category, Tags and Taxonomies) data/terms. The Search & Filter Pro Text Search field uses the standard WordPress search, so looks only in the Post Title and Content. To help you control this better, on the form’s Advanced settings tab you will see 2 settings for Relevanssi.

The documentation for this is here:

https://www.designsandcode.com/documentation/search-filter-pro/3rd-party/relevanssi/

You would need to install and activate the free Relevanssi plugin as well.

Then set Relevanssi up and build its index (make sure it is indexing the desired post types, custom fields AND any taxonomy that you want to search).