BlueCoat Proxy authentication issues

So, I am having an issue with some of my end users authenticating to our proxy servers.  From time to time I will get a call because they can't access a web page that they should be able to (the pages are not blocked.) What's happening is that they are authenticating to the proxy with their machine account instead of their domain account for some reason.  To fix this issue, I just log out their machine account on the proxy server and then their browser will re-authenticate with their domain account.  It keeps happening more and more and its now affecting multiple users, multiple times throughout the day.  Any ideas on how I can resolve this issue or even what is causing it?  

Thanks for your help!!