Network users not appearing on new Yosemite iMacs.

I manage a small school iMac lab (about 30 computers). We use Workgroup Manager 10.8 running on OS X server 10.8.5 to manage our users and groups. I recently installed 10 new iMacs running Yosemite. Once these new computers were joined to the network, there were a small number of users whose login accounts don't appear at the user selection screen. The users are there on the older computers, which are still running Mountain Lion. If I choose "Other" and manually type in the user account information, I can log in with those users, but they still don't appear on the user list once I log out. Why would the users show up on the older Mountain Lion iMacs but not the newer Yosemite iMacs?