Can't connect to friends server

Hi, I am trying to connect to my server but it will not let me it does not give any error it just says ''Can't connect to server'' or sometimes it gives some Java error, my friends are able to connect to the server but I can't, I have done the DynIP, but I don't want to ask for it every time, my friends are able to connect with the main IP of the server, but for some reason I can't and I have to use the DynIP if I want to connect, is there a way to fix this so I can connect with the same IP every time?