People are unable to join my server

So I recently detected a problem. Eventhough my server is on the updated version of Minecraft and people's Minecraft versions are updated and they've entered the clrrext IP and Port. They are unable tp join my server.
Sometimes myself experience that I have to click on my server button multiple times in order to join, because it keeps failing to connect to my server.
I have tried the basics like restarting, changing the server slots, repairing, roleback, etc... Nothing seems to be working.

Ps. Sorry for creating so many threads. I'm judt trying to report as much bugs as possible because its really a problem once you want to prepare a decent server for players to join.