Could their ITIN's have expired? 

Individual Income Tax Identification Numbers (ITINs) Expiring--All ITINs that have not been used on a federal income tax return in the last three years or ITINs issued before 2013 with the middle digits of 70, 71, 72, or 80 (9XX-70-XXXX) will expire on December 31, 2017. For more information visit the IRS website. ITINs are issued to taxpayers that do not qualify for a Social Security Number.

This comes from Indiana Dept of Rev website.