Mac OSX 10.7 server with 10.6.8 clients - old network accounts work, now ones created since May do not

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http://serverfault.com – I have a Mac OS 10.7 server, and a number of 10.6.8 clients in a school. For the last year and half, network accounts could log into the clients with no problem. There is a set of presets in Workgroup Manager for correct default Group (staff vs students), home directory path, etc. Since about May, all new accounts cannot log into the 10.6.8 clients. Old accounts (created pre-May) still can, and not just with cached credentials (clients wiped and restored). An OS 10.7.4 client CAN log in with the new account, just not the 10.6.8 ones. I have checked all of the usual subjects (DNS issues, (HowTos)