Schema data can’t see custom fields

Setting up schema, trying to pull address data etc. from posts.

I can’t access any relevant custom field data.

In the Add New Schema screen I select “select your custom field”, which populates a list of custom fields. None of the custom fields I’m using appear in the list. Perhaps because they begin with an underscore? eg. _cmb_street_address ?

Is there anything I can do to get this working?