Have you tried to set the server to static IP address?
Have you tried to map the connections using computer name instead of IP address. (you didn't answer how this was done)
Have you setup a homegroup and made all the computers members?
Make sure that you have enabled file and print sharing on all the computers.
Have you checked the permissions on the shared folder so that the group "everyone" has access to them?
How are the computers connected? A router or switch or something else?
Can you go to Start>Run>\\server name?
Check the permissions to see if there is a limit on the 'con-current' connections (i think that's what it's called - where it limits how many people can connect at once). Not sure if this is an issue or if it's something you can set in Win7, but I think it might be worth looking into.
I can't say for sure that formating the computers will solve the problem.
A quick google search brought this up: Joining Windows 7 Homegroup breaks connections to shares - Super User
. Don't know if it will be of any use.