ssh-agent could not add signed cert when private key stored in yubikey

[YC]

Hi,

I'm currently stuck with yubikey + signed user key + ssh-agent forwarding.
As https://developers.yubico.com/PIV/Guides/SSH_user_certificates.html 
noted, I have private key stored in yubikey, public key in 
~/.ssh/id_rsa.pub and signed public key in ~/.ssh/id_rsa-cert.pub on PC 
(see bellow).

It's not working with this agent forwarding access: 
PC----Server_A----Server_B. Placing
private key saved in ~/id_rsa, it works fine! After a simple comparsion, 
I found that when
private key store in yubikey hardware, ssh-add would not add signed 
public key (id_rsa-cert.pub) to ssh-agent, should this be the problem? 
Is there a way to add signed public key to ssh-agent?

-- 
yc.