You need to describe, in detail how all your hardware is networked together. Once you do that I can tell you a good idea of what is wrong.
What kind of cable comes out of your Amiga ethernet port?
What does it plug into? (pc or router or ?)
How many network cables are plugged into the PC?
What is your Amiga's IP address?
What is your PC's IP address?
Are you 100% positive that ALL your PC firewalls are off?