My bad, I knew it was trivial – found answers in http://www.advancedcustomfields.com/resources/working-with-nested-repeaters/ – didn’t make the link between nested repeaters and flexible content.
To precise, I do have ” – ” displayed which means it’s not about the if statement. Also, the structure of the flexible content custom field is as follows:
field name: post_quote_field
layout name: post_quote_block
field_name: post_quote
field_name: post_quote_author
field_name: post_quote_source_link
