Troubles with Cisco 2620 Router?

I have a IPgateway (NGS4000)- internet subscribe server II (i named is ISS). It has 4 LAN ports and 1 WAN port. It has authentication (like Radius) built in function, it worked okay. ISS will ask user name and password all users through it. When i used it with a Cisco 2620 Router and Cisco 2950 Switch to routing all PC to ISS, it has a strange problem. At the first time, one PC from Switch 2950 go to ISS (after routing VLAN with Router 2620), ISS asked password, that is no problem. But when the second one through out ISS, it no ask any more(same with later PC). It seem ISS think all destination IP address (of PCs) from Router2620 is have a the same IP (interface 0/0 of Router has any IP address). Is this a problem from Cisco device? Thanks for attention!

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