How to grant drop table/function privileges to a role in PostgreSQL?
Only the owner (and superusers) can drop objects. Per documentation:


  The right to drop an object, or to alter its definition in any way, is
  not treated as a grantable privilege; it is inherent in the owner, and
  cannot be granted or revoked. (However, a similar effect can be
  obtained by granting or revoking membership in the role that owns the
  object; see below.) The owner implicitly has all grant options for the
  object, too.


So, make administrator own such objects that users should be able to drop.

ALTER FUNCTION foo() OWNER TO administrator;
ALTER TABLE foo      OWNER TO administrator;


And you remembered to actually grant group membership, right?

GRANT administrator TO _administrator;