I'm running a forge server for 1.12.2 that has quite a large mod list.
I can join it just fine, and so can some of my friends, but others can't join at all because the server refuses to recognize the mods they have installed.
They have all the correct mods as they all show up in the main menu mod list, however the server won't recognize a single one of them and they're unable to join because of it.
Get the latest.log from one of them and add it with paste.ubuntu.com
can't do that right now because they're all asleep but I doubt it would help because as I said they can use all the mods in singleplayer, it's just when joining the server that there's an issue
The issue should be shown in the log-files
All it says is that "x mod is not found on client" for each mod installed
Maybe test it with another client or make sure the forge-version is the same