webrating not working in transparent mode

Hello

I have set my my fortigate in transparent mode so it can connect in the networking without any changes and work as a web filter for me. 

however whenever apply a web filter in my policy the fortigate starts droping the packets and the reason is that it is not able to do the web rating for any URL. 

I have tried to do the same when having NAT mode and it worked perfectly I have changed nothing in terms of connectivity to the ISP and the DNS and it worked. 

so is there any limitation or something extra that is needed to be done in transparent mode?

thanks, 

Hassan