If a line item provides an incentive for on-time delivery and the contractor fails to meet the delivery date stipulated in the contract, then it's a matter of whether the missed delivery date is due to fault of the contractor or action/oversight by the Government. If the former, then the contractor has not earned the delivery incentive payment. If the latter, then that constitutes a constructive change. In that case, the modification issued to update the delivery schedule should mention (in block 14 of the SF 30) that the delivery incentive is tied to the revised delivery date.