Someone with experience of Rackspace Windows Server 2008

I have provided my client with a temporary solution whilst I develop a new totally integrated solution.
I have provided a Windows 2008 server supplied by Rackspace. My client has up to 6 users each of which have between 1 and 3 MS Access front ends. The Back Ends are located on the Windows Server.
Each user has been asked to Map their A: Drive to the appropriate folder on the server.
The problem I have is that some users have successfully mapped and connected. And some just quite simply cannot map. They get "Network not Found" error message. Personally I am finding that sometimes I am connected and other times I am not.
I had a long "Live Chat" with Rackspace today and whilst on I could not Map the Drive. However they did Map the drive from two different PC's.
Could someone please shed some light on this for me.
I understand this will only be a token payment for you but I will be very grateful to the person who can crack this for me and hopefully I can use your expertise again in the future.
Many Thanks,
Carl.