authentication

I have an active server, where i have a few shared folders, with each user on the active directory having different permissions. many of the users are connecting to to the server properly; authenticating every time they attempt to access the share(via START-RUN-server IP- OK)....however, some of the users are not being asked 2 authenticated anymore....when they START-RUN-server IP- OK, they simply have the network share folders on their screen....the problem is; they cannot access the folders, even when they have permission........plz help as this critical....