c # - x509 certificate does not recognize it in windows server 2008 R2

I'm trying to access the certificate store on a 2008 R2 server and it says

clientCertStore.Certificates.Count = 0

The same request with the same certificate in my localhost if you return it to me, the only difference is that my PC is a W7.

The installation of the certificate has been done in the same way on both machines, with an exportable key and in the Personal store.

I do not know if it will have to do, but the certificate is expired, but as I say, on my PC if you return it to me.

private X509Certificate2 Certificate(string nif) 
{
    X509Certificate2 cert = null;
    X509Store store = new X509Store(StoreName.My, StoreLocation.CurrentUser); 
    store.Open(OpenFlags.ReadOnly); 
    X509CertificateCollection certs = store.Certificates; 
    int c = certs.Count; 
    foreach (X509Certificate2 cert_aux in certs) 
    { 
        if (cert_aux.SubjectName.Name.Contains(nif.ToUpper())) 
        {
            cert = cert_aux; break; 
        }
    }

    store.Close(); 
    return cert;
}