Having installed QB's in many of my different clients networked environments I would like to add a few things from my own experience to assist with this forum members problem and perhaps help anyone else who runs into the same or similar issue with QB.
It is not correct that QB requires you to MAP the drive. Actually, mapping the drive for some reason actually can cause conflicts with in the QB database manager, especially if you have multiple machines in the environment (more than two that are using the qbw file). This is because often people will create a map using drive:x on machine 1, and a mapped drive:Y on machine 2, and so on.
The trick to an almost flawless installation and networking of QB's is to download the QB File Doctor from Intuit and 1st run it on the machine hosting the file, then run the same program on each machine in the network which is attempting to access the file across the network. This program from Intuit, will open all the necessary ports in your firewall to allow the proper sharing of the qbw file and will also typically create the necessary shares on your hard drive. I now make this my normal routine on every network installation and I've had zero issues since incorporating it into my installations.
One other caveat I would like to share: AVG antivirus (and possibly a few others) need to have exceptions added to their programs. (unless this has been corrected since my last installation about six months ago.
Hope this helps someone.