QuickBooks 2011-2012 occasionally will not accept an IIF file. Instead it will give you multiple line errors and tell you that your data was not imported. To solve do the following:

  1. Go to Lists > Other Names
  2. Find your Website Name - It is usually all capitals for instance CANKISU Website.
  3. Delete your customer name
  4. Go to Customer Center
  5. Select Add a new customer
  6. Enter in your website name exactly how you found it in the 'Other Lists'
  7. You will not need to enter any other information
  8. Import your IIF again

See Also
IIF File was imported but I don't see the receivables/invoice in my reports (Canadian date issue)
Why is my QuickBooks IIF file showing a negative balance?