You must file the loss return to carry any net loss to another tax year; however, if the loss return is filed more than three years after the extended due date and also calculates an overpayment, then your overpayment will not be available to be refunded or applied toward a subsequent reporting period.
EXAMPLE: You claim a Net Loss Deduction on your December 2017 Form IL-1120. The loss you are claiming is from your December 2010 reporting period; however, you did not file your December 2010 Form IL-1120 until March 2017. Your December 2010 Form IL-1120 calculates a net loss and claims an overpayment from your December 2009 reporting period which you request as a refund. The loss generated on your December 2010 return is available to be claimed on your December 2017 Form IL-1120; however, the refund claimed, due to the overpayment from the December 2009 reporting period, is unavailable to be refunded or applied toward a subsequent reporting period because your December 2010 reporting period was filed more than three years from the extended due date of the original return.