Lync client can't fetch the address book files

Hi,

We have 3 FE Lync 2010 servers. All the clients are giving an error "cannot synchronize with the corporate address book".

I can access Internal Web services URL and download the AB file.  I have also made an entry for Lync Internal web services in the host file and signed into the Lync. Then the client was able to download the Address book files successfully. Any ideas to resolve this.?

Thank you !