I had a similar problem where other users could see the share and files on the share, but couldn't write to existing files - they could create new files. In my case it was an ownership issue - root had copied all the files to the share.
To fix this, I created a group (named pco), then used chown and chgrp to "give" those files to the group, then in smb.conf, in the section for the share, I set
valid users = @pco
again - I am very new to all this, so not sure if it's relevant to your situation, but it solved my problem.