There is much about your situation that we do not know and which might help us to identify your issue. But this part we do know: the selection of ports is defined on the caching device (websense in your case) and associated with a service number which is configured on the Cisco device. So the first step in troubleshooting would be to verify on websense that it has specified TCP 91 associated with service group 70.
If you check on websense and it does specify TCP 91 associated with service group 70 then the next step would be to share with us the config of wccp on the cisco device.
HTH
Rick
HTH
Rick