how to debug permission denied errors

I need to access several Windows domain shares from my linux box. I can access both shares from 
a Win box where I use my domain login. I placed my domain credentials in /home/przemek/.smb_credentials 
and I can successfully mount one of the shares:

  mount.cifs '//elwood.nist.gov/61_NCNR/610'  /mnt/ --verbose -o uid=przemek,credentials=/home/przemek/.smb_credentials 
    parsing options: uid=przemek,credentials=/home/przemek/.smb_credentials

    Domain NIST

    mount.cifs kernel mount options unc=//elwood.nist.gov\61_NCNR,user=przemek,domain=NIST,pass=mypass,ver=1,credentials=/home/przemek/.smb_credentials,uid=500,prefixpath=610 

  df
    //elwood.nist.gov/61_NCNR/610  21893480448 14847986400 7045494048  68% /mnt

but not the other one:

  umount /mnt

    mount.cifs '//ociosfs.nist.gov/plan$/NCNR'  /mnt/ --verbose -o uid=przemek,credentials=/home/przemek/.smb_credentials 
    parsing options: uid=przemek,credentials=/home/przemek/.smb_credentials

    Domain NIST

    mount.cifs kernel mount options unc=//ociosfs.nist.gov\plan$,user=przemek,domain=NIST,pass=mypass,ver=1,credentials=/home/przemek/.smb_credentials,uid=500,prefixpath=NCNR 
    mount error(13): Permission denied

How could I check what is going on? Is there more info to collect on
the Linux end on why it's denied? Are there questions to ask the
Windows domain/server admins?  The two shares are on different
servers, and the failing one is a hidden share (plan$), but as I said,
they are both simply visible within Windows.