You can check to be sure you don't have an other program that is using the ports you use, and turn off firewalls/anti virus if you have it on. Other possibilty is a problem with your internet. Even tho your internet can appear working fine, there can still be things wrong with it. I had this problem myself a few times, one time it was a cable, 1 time it was my modem (needed to be replaced) and like 3x it was a problem with my internet provider (which was solved automaticly after about 2 weeks). In most cases it's a problem with ports or firewalls/anti virus programs, but if it might be your internet, you can always try to restart your router/modem and check if there are any reports on the internet about problems with your internet provider.