If you notice that a Programmatic Guaranteed deal in Display & Video 360 under-delivered last week, the first step you should take is to use the Deal Troubleshooter.

• See if the line item has multiple targeting assigned.
• Review the campaign-level targeting.
• Contact the publisher.
• Use the Deal Troubleshooter.

The correct answer is: Use the Deal Troubleshooter

Explanation: The Deal Troubleshooter in Display & Video 360 is an essential tool designed to analyze and identify issues with various types of deals, including private auctions, guaranteed deals, non-guaranteed fixed deals, and open auction line items. It offers a straightforward overview of why a deal may not be spending as expected. This tool is compatible with deals from any exchange integrated with Display & Video 360, allowing for specific troubleshooting, especially if the line item targets multiple deals. The Troubleshooter provides detailed insights into the reasons behind lost impression availability and why a line item might not be securing available inventory, offering quantifications at three different levels to comprehensively understand and address the under-delivery issue.

Read more here: https://support.google.com/displayvideo/answer/6292894