No response for WinHTTP API calls

Question

My application uses a PAC file to determine the proxy to be used for a given URL.

It uses WinHttpOpen, WinHttpCreateProxyResolver and WinHttpGetProxyResult Apis to get the proxy info.

I am encountering an issue where I find that the registered callback is never called. I do not suspect the implementation as it works most of the time and when it doesn't work I find it hung waiting for the callback from Windows.

What could be the reason for WinHttp layer not calling the callback function ? If do not see any errors in the event logs.