SSL VPN Authentication with Cert (Fortigate) - works on a Mac but not Windows?

I have the CA and server cert installed on the Fortigate and I install the user cert on my Mac (and require client certificate on the VPN settings of the Fortigate).  It works.  However, the same user cert on a Windows 7 machine does not.  I get the "The server you want to connect to requests identification, please choose a certificate and try again. (-5).  I know the setup/config works but it's almost as if the Windows machine isn't presenting the cert to the Firewall even though I select the cert int he drop down of the Forticlient login.  Any ideas?  Thanks in advance...