Try the most basic things like re-compiling the mission from the editor, and also try running it LAN on a server then if that works try using it over the internet.
Also double check if you are hosting the server correctly, (If your hosting it yourself, make sure the port for arma 3 is open on your internet.) If you are paying for someone else to host the server for you like gameservers.com then try reinstalling the mission on that. Sometimes asking for higher priority on the servers (Only when paying for servers.) can help, if there is an option of course, but it will also cost you a bit more money.
It's probably easier to just reinstall the server if its not working when you try hosting it, close the port, then re-open it, try everything you can. Hope this helps! :)
