The JSON value is inserted into a column in the database which I can see. But the trigger which has to convert this JSON value in not tsvector column is not updating that column.
As Tom explained, in your trigger function you have:
Well, the obvious comment on that is "that JSON value hasn't got any qtext field". So the ->> operator is returning null, the coalesce() is replacing that with an empty string, and you end up with an empty tsvector column.