Hello, I have a HP Officejet Pro 8600 Plus N911g installed as a network printer on my Windows 7 64 bit Home Premium. I ran the HP Scan Diagnostic, and at first it failed the WIA Scan test, but the printer was at idle. So I selected Scan in the touch panel to sort of wake it up. Then it passed WIA Scan, but went on to fail the HP Twain Scan. I checked my software and drivers and they are current. As a side note, when I click Start > Devices and Printers, it takes two minutes to open the window, so some of this could be timeout issues. All suggestions gratefully received!
It does sound like you are having network connectivity / delay issues. Have you ran the HP network diagnostic utility to see if it gives any help?