Shared folders not visible if FQDN is not used

Shared folders on one particular Windows 2003 file server are not visible from time to time.

For example, I have
“net use f:\server1\docs” in my login script, I can access f drive but no contents. If I use UNC, \\server1\ from start, run, the shared folder docs would not display at all either.

HOWEVER, if I use FQDN such as \\server1.domain.local\, everything works properly.

I guess it has something to do with this server hosting users’ redirected “My Document” folders. No problem with any other file servers at all.