Mac / Server 2003 sharing

Here's a head scratcher ...

We have a mixed platform environment. We have set up a new Server 2003
server and made it available to PC and Mac users. All works fine. Because
some of the Macs are using OS9 we have had to allow them to log in using the
guest account.

If a file is placed on the server using a mac then the pc users can access
it. If a pc user places a file on the server then the mac users can't see
it. If we delete the share and reshare then all the previously 'invisible'
file are available to the macs.

Anybody able to help?

Thanks

Colin