NT Roaming Profiles

Our users are meant to be able to login, have their login script run and pick up their computer’s profile from the main file server. Things were fine until, for no apparent reason this stopped: for most the login script runs but the profiles are notalways downloaded, eg. the standard desktop has stopped loading completely. Where do I start trying to rectify this? The server runs NT4 Sp6, clients are using NT4 Wkstn Sp6 and Win2000. Thinking about it, the problem only became apparent after the introduction of Win2K clients to the network.