While this is probably the most common issue, it is also one of the hardest to troubleshoot. One of the downsides of QuickBooks is that it can be rather fussy when it comes to network connections. If there is any issue with the network, even minor, it is possible for the connection to be lost. Start by ensuring that QuickBooks is set up correctly and that the firewall is allowing the connection. Next, use the QuickBooks connection diagnostic tool to determine the issue.