Is your web browser pointing to the new proxy server?
On the proxy server, run:
# watch -n 1 'netstat -tn | grep 3128'
Do you see any activity in that netstat session when you try to visit a web page? If not, double check (again) your web browser settings. If those look OK, run the following from your workstation:
$ nc -zvw 1 proxy.host.here 3128
What does that tell you? If it times out, something is likely filtering traffic to that host/port.