I meant that setting up a server on more recent systems, including El Capitan, is more difficult than it was. I've gotten it working on 10.12.6, so there shouldn't be any existing OS that it doesn't work on.
But since your system did work, I was questioning whether or not there was a change on the OS too. If so there's been a lot of info shared here previously on how to work on the server configuration. I wasn't going to repeat them unless there was reason to do so.
If the server has been unchanged, there are other places to start.
Do your client computers detect the server in the Available Servers Wizard? If not, how about when you turn up the Bonjour Threshold: