User called me and told me the other day that even though their multi user was setup properly in QuickBooks, they couldn’t actually have the file open on two computers at once. Everything looked fine in QuickBooks, only one computer was hosting multi-user (in this environment it was decided for it to be hosted from a desktop and not the server), etc.
I was very surprised to find a great tool from Intuit to help solve networking problems. It properly identified the configuration needed for the firewall and fixed it. Just run the Network Troubleshooter installed with QuickBooks and see if it solves your problems first.