DOIs Do Not Work

Dear Sir,
I have published four issues. The first and second issues show their DOIs and are properly working, but the third and fourth issues’ DOIs are not working. Please let me know how to solve this problem. I am using the OJS system.
Thanks

Hello @niazali ,

Thanks for your message, and welcome to the community forum. We’re happy to help. Could you let us know what your username and/or DOI prefix is so we can take a closer look?

Thanks in advance,
Isaac