If their connection attempt doesn't even register in teh server log it means that it's being blocked before it even reaches you. This means that a firewall or router is denying the connection.
1 ) Deactivate your firewall completely and have people try to connect. If this changes nothing then it must be your router.
2 ) If it's your router then you'll need to forward your port. I know you said you did, but trust me it's hellaciously easy to bollocks that process up.
http://portforward.com/
Is a decent resource for finding a port forwarding guide for your router specifically. Or you can just google the manufacturer and model number.
Very generally, forwarding a port entails: naming the exception, telling the router which port wil be forwarded, and then where it will be forwarded to. You'll need to know which port your server is using (shoudl be 25565) and then your local IP under the router. It fou go into the command prompt and enter "ipconfig" it should be your ipv4 address.
If this doesn't work then I suggest finding a nice outdoor hobby, cause that's all I got.