erratic behaviour of Workgroup Manager

Maybe I miss something obvious but here is my problem : I use Tiger Server (10.4.9) with OD for 10 users. It works fine. My only concern is that from time to time when I log in as diradmin in the Workgroup Manager I can't create new users (New User button is grayed out) and I have only a limited access to the user profiles (basically I can just change the passwords, all other fields are grayed out). Any suggestions ?