Mutual SSL for Web Service in IIS 7

Hi,

I required SSL Settings in my web site, when client call it, it will ask for Certificate Request, this is working fine,

however, when they present Certificate, they received forbidden access 403 from my server.

For your information, their certificate are already installed in our Server Trusted Root Store.

Is it related to CA Store? Kindly advice if any steps.

my web service are normal ASMX, hosted in isolated web instance for this client.