You’ve implemented a WPAD.DAT auto detect proxy file however some clients or sites are not connecting to it.
Check the the there is a DNS entry called ‘wpad’ which points to the server which is hosting the file. (http://wpad/wpad.dat)
Check that the clients can ping ‘wpad’ and it resolves to the correct host
By default on Server 2008 the global query block list is enabled. This stops the wpad from being available through DNS.
To disable run the following command on all DNS servers:
dnscmd /config /enableglobalqueryblocklist 0