It seems that every time a certain client of mine (at two different businesses) updates their quickbooks POS (point of sale) system, they lose connection to the quickbooks data and can't synchronize their transactions from the POS to Quickbooks.
The instructions at http://support.quickbooks.intuit.com/support/articles/SLN48142 and http://support.quickbooks.intuit.com/support/articles/SLN40703 are of no use, but I have learned a few things after spending time on the phone with their tech support.
Run Quickbooks on the POS station in single user mode. Select edit, preferences (from the menu) and then click on Integrated Applicaitons. Click the Company Preferences tab at the top of the window. You should see Point of Sale listed in the box. Click on it, then click the remove button.
Leaving Quickbooks Running in single user mode, run the POS system and select edit/preferences/company (from the menu.) Click on Financials and then click the checkbox "Use with Quickbooks Financial Software*" Then click setup Quickbooks connections Now to connect to Quickbooks. If it says connection successful, you are done.
If this doesn't work. Do it again, but quit quickbooks and POS between steps.
It seems that POS (kinda unintuitively can post to a Quickbooks file while Quickbooks is running in single user mode)