Proxy Server Question

I would apreciate some help.

I have a proxy server which is connected to pc’s and iMac’s.

The users can use the PC’s without any problems, however when the same users use the iMac’s to surf the internet, they cannot.

I have configured the iMac’s correctly to access the internet. All the settings are correct, and I (with administrative privilages) can view pages.

So, what is the problem?

If you need more info, just ask.