Trouble joining servers

Hello. So lately, I have been having trouble adding new servers that I have already played, and have decided to play again. So, for example, I add a server that I have played on before, exact IP, exact port, all that stuff. Then, it is loading, it doesn’t load, and displays: “Unable to connect to world.” It is super annoying, even considering that I can still join the default servers. Please fix this.