2nd Pc Connection Problem

I have a Sitecom router and have a Dell Dimension 5100 connected by cable which works ok  I then connected another Dell Dimension by a cable upstairs he router shows that the 2nd computer is present but am unable to access the internet. I have swapped computers but the computer on the Longer cable will not connect to the net I have replaced the cable ?? We have run a Cat5 cable from the router through the house to another room This we have done twice We have swapped pcs around and they work on the short cable but not when connected to the longer?? We have treble checked the cable and all connections are ok The Router is a sitecom WL 322 that identifies the pcs and is working fine. We have tried altering the IPs without success  Can you help please  Nigel