More Samba questions...

I know you guys are probably getting tired of me and my Samba issues, but I
really appreciate you all bearing with me...

First, a couple questions about how Samba stores users... How does this
work? It's my understanding that Samba users are stored in the smbpasswd
file. However, I've run into a situation where a user exists in that file,
but I'm unable to connect to shares with this user nor can I change the
password (smbpasswd user) until I first run `smbpasswd -a user`. (This
changes nothing in the smbpasswd file, leading me to believe the "allowed
users" is kept elsewhere, but I'll be darned if I can find where.)

On to the next issue; connecting to shares. Using the newly added smb user,
I am able to connect to shares from WINDOWS machines only. If I try to
connect to the Samba server using the same username/password credentials in
Ubuntu, I get stuck in a loop where it keeps asking for the password and
never accepting it. (Ultimately, I cancel and it throws an error about not
being able to display the contents.) From this same Ubuntu box, I AM able
to connect to shares on WINDOWS machines without a problem. It's only when
I try to connect to Samba shares on linux machines does it fail.

Can anyone tell me what I might be doing wrong in either situation?