Postgresql: Trigger is not working at times

I have a PostgreSQL trigger that is not firing sometimes, even though the status is always shown as "enabled". My trigger code is as follows:

CREATE OR REPLACE FUNCTION audit_src_exhibit() RETURNS trigger AS $BODY$
BEGIN
  IF TG_OP = 'INSERT' then
		if new.audit_created_date is null  THEN
		new.audit_created_date := current_timestamp;
		new.audit_created_by := session_user::text;
	end if;
	else
		if new.audit_modified_date is null  THEN
		new.audit_modified_date := current_timestamp;
		new.audit_modified_by := session_user::text;
	end if;
END IF;
  RETURN NEW;
END; $BODY$ LANGUAGE plpgsql VOLATILE;

CREATE TRIGGER audit_src_exhibit_tr
BEFORE INSERT OR UPDATE ON 
FOR EACH ROW EXECUTE PROCEDURE audit_src_exhibit();

Is there any specific reason for this behaviour? Does my code show any signs of known issues which would result in triggers not firing? I am getting the audit columns as empty even though some insert and update happened today