[Samba] Users cannot loggon domain

[ROUMPEKAS XRHSTOSNEKTARIOS]

dear friends,

I am faced with the following problem:

I have a Domain created in Samba 3.0.28a (Ubuntu Server 8.04) where there are 4 PCs. 2 of them are operating absolutely fine.
the other 2 however when they loggon the server with their username/passwd credentials are prompted with a message from Windows telling them that there is an error in their password, domain etc.

i then alter their passwd with smbpasswd username and retype their existing password. At this stage the users can loggon successfully.
If they switch the machines off and on and try to loggon, I have to redo the same process with smbpasswd in order for them to loggon.

the workstations are WinXP SP2 and have the requiresignorseal changed as per the instructions of Samba website.
This is very odd as 2 out of 4 machines never have this issue and the process to join these machines to the network is identical for all of them.

Any good ideas?
Password system is smbpasswd.

Chris