Security Authentication Certificates Issue

I am attempting to test a webservice that is up and running on our network. I have endpoints to use and parameters in order to send requests to the service. I have certificates and I have installed them to my Windows certificates and then I have configured the SSL preferences to point to the certificate. However when I try and send any request to the service I just get the same response back which is:

HTTP/1.1 403 Forbidden

Is there something I am not doing right ?