Have you tried joining a windows workstation to the domain, then logging in as bob, and then tested to see if you can then access the share?
I haven't tried accessing my shares with Ubuntu yet. bob is a LDAP user. Typically, on a windows machine, by logging into windows as bob, the share knows you are bob when you try to access the share.
You said you are using bob's credentials, but I wonder if knows you are bob of the correct domain name. Try using yourdomain\bob for the username, maybe that will help?
Also, just to ensure there's not some crazy quota limitation, set bob's quota to 0 (zero) during your tests.