First, ensure that all of the ports that you have assigned for DCC connections are open through the router. If you're not sure how to do this, check the documentation that came with your router and/or the manufacturer's website and/or
PortForward.com If everything there is correct, and you're still having problems, contact your ISP and ask them if they're blocking the ports that you have specified. Note: You may have to inisist on speaking to a technician, as the people that usually answer the phones just relay what they are told, and may not have accurate information.