One of my players appears to be recieving this issue. When they try to join they recieve an error saying that the server is not responding. He is the only one recieving this issue. The server is updated and all the mods have been updated as well. He also uninstalled and reinstalled all his mods and his game multiple times and is still recieving the error. We cant seem to figure out what the issue is. 

There are a few things that will cause this error:

Anti Cheats can cause this for people that may have programs loaded in the background causing additional network strain.

Netowrk issues between the player and the server may cause the error message.

The players PC may not be procesing the local files fast enough.

If you have space for a test server, set one up on the same network as your main server. (A, B or C) 
Make it vanilla. Test. If they can't get on, then you can then turn off the AntiCheat.
Test. If they can't get on then, you can presume it's a network problem from point A to point B.