iCloud Keychain Fails to Sync after update Mac OS X Yosemite

Apple suggests to switch off iCloud Keychain on all devices that use it. Then, figure out which of your devices has the most up-to-date list of passwords and switch on Keychain for that device. And finally, turn on iCloud Keychain for all other devices. This should cause Safari to produce the passwords you’re looking for.

Apple outlines the steps for doing so in Get Help Using iCloud Keychain document:
iCloud Keychain isn’t syncing with other devices.
Make sure that your device is on a Wi-Fi or cellular network. If the connection appears strong, try these steps:
1. Turn off iCloud Keychain on all devices.
2. Turn on iCloud Keychain on the device that has the most up-to-date keychain items.
3. Turn on iCloud Keychain on your other devices. They will be updated with your new iCloud Keychain.