smack 4.1.7 Vcard return null, while ProviderManager add vcard IQ

I have add this line:

ProviderManager.addIQProvider("vCard", "vcard-temp", new VCardProvider());

after

xmppConnection = new XMPPTCPConnection(config);

but when I call these line:

VCardManager manager = XmppManager.getManager().getVCardManager();

if (manager.isSupported(entry.getUser())) {//return false

  vCard = manager.loadVCard(entry.getUser());
}

//or directly call vCard = manager.loadVCard(entry.getUser()); //return null

any help will be grateful。

At last, I would like to say sorry about my poor English , I can read but I can't write~~~