Some news servers insist on rewriting the message id in your upstream articles. This means that scoring follow-ups by scoring on the message id in your outgoing article (gnus-score-followup-article, gnus-score-followup-thread) won’t work. You still can score on subject, but this is not always what you want.

The following has nearly the same effect as scoring on your message id directly:

    ;; Increase Score for all follow-ups to my own articles (can't
    ;; use message-id for scoring since it is changed later by
    ;; my news feed)
    (defun hcz-gnus-score-followup (&optional score)
      "Add SCORE to all later articles in the thread the current
      buffer is part of.  This version is for cases where the own
      message-id will later be rewritten upstream.  It scores on the
      message-id of the parent article (which has nearly the same  eff
      ect but also scores parallel replies).  If there is no
      parent article (we are opening a thread), score on subject is
      done instead."
      (interactive "P")
      (setq score (gnus-score-delta-default score))
      (when (gnus-buffer-live-p gnus-summary-buffer)
            (goto-char (point-min))
            (let ((refs (mail-fetch-field "references")))
              (if (and refs
                       (string-match "\\(<[^<]+>\\)\\'" refs))
                    (set-buffer gnus-summary-buffer)
                     "references" (format "%s" (match-string 1 refs)) 's
                     score (current-time-string)))            
                ;; if we didn't find a reference (probably due to no
                ;; parent article), we increase the score on subject:
                (let ((subj (mail-fetch-field "subject")))
                  (when subj
                    (set-buffer gnus-summary-buffer)
                     "subject" subj 's
                     score (current-time-string))))))))))
    (add-hook 'message-sent-hook 'hcz-gnus-score-followup)