This is a follow-up to an earlier question about a case of blatant self-plagiarism, following which I contacted the editor of the concerned journal.
The editor did not acknowledge my initial report. It took a polite follow-up email about a month later before I got an answer saying that they were looking into the report and that they would let me know what they found. Since it is a blatant case, I expected that the investigation would be over pretty quickly. After I hadn't heard anything for about another month, I sent another polite email to ask about the status of the investigation. Another month has passed since then and still there has been no reply.
I get the impression that the editor does not appear to pursue this case. Of course I know that the editor must be very busy, but I do not think it is unreasonable to expect something to have happened more than 3 months after the initial report.
I tried to get in touch with the managing editor of the journal, but was told that the publisher will not provide the contact information. To put it mildly, I find this all very discouraging. I expected that reports of self-plagiarism would be taken more seriously despite the offence not being as serious as plagiarism or fabrication.
What should I do next?