yubikey-agent is a seamless ssh-agent for YubiKeys.
If other cards, e.g. due to internal card readers, are present, they are probably preferred and the YubiKey can not be used. Ignore unready cards and use the first available one. Internal card readers are often present but seldom used. But they probably show up, at least that's the case on my system, before the external YubiKey and hence prevent its usage for yubikey-agent. When using the first ready card chances are much higher that it's actually the intended YubiKey.
This issue appears to be discussing a feature request or bug report related to the repository. Based on the content, it seems to be resolved. The issue was opened by bertschneider and has received 4 comments.