The following works for me:
public async Task<X509Certificate2> GetCertificate(string certificateThumbprint)
{
var store = new X509Store(StoreName.Root, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
var cert = store.Certificates.OfType<X509Certificate2>()
.FirstOrDefault(x => x.Thumbprint == certificateThumbprint);
store.Close();
return cert;
}