Sessions

I have an NT 4.0 network. A PDC and one BDC.
The PDC keeps holding sessions open after users disconnect (they either shutdown without logging off properly or they use Terminal Server and disconnect their session)
These sessions left open prevent new users logging in from accessing the NetLogon Share and thus getting thier policy and drive mappings. Any idea why the PDC isn’t dropping these users? Some are connected and idle for over 500 minutes